GEMARA

(Mishnah): If the Sanhedrin forbade idolatry, but exempted one who bows...

They forbade spreading the hands and feet while bowing. They permitted bowing without doing this.

Megilah 22b - Question: When Rav came to Bavel, all fell on their faces, but Rav did not. Why didn't he?

Answer #1: There was a stone floor.

(Beraisa): "V'Even Maskis Lo Sitnu b'Artzechem Lehishtachavos Aleha" - you may not bow on your stone floor, but you may bow on the floor of the Mikdash.

(Ula): The Torah forbids only spreading the hands and feet.

Answer #2: An important person is different.

(R. Elazar): An important person may not fall on his face unless he is sure that he will be answered, like Yehoshua - "va'Yomer Hash-m El Yehoshua Kum Lech..."

(Beraisa): "Kidah" is (bowing) on one's face - "va'Tikad Bas Sheva Apayim Artzah". "Kri'ah" is on the knees - "mi'Kro'ah Al Birkav." "Hishtachava'ah" is spreading hands and feet - "...Lehishtachavos Lecha Artzah".

(R. Chiya bar Avin): I saw Abaye and Rava fall on their faces and lean on their sides.

Rishonim

Rif (Megilah 13a): R. Elazar taught that an important person may not fall on his face unless he is sure that he will be answered, like Yehoshua. R. Chiya bar Avin saw Abaye and Rava fall on their faces and lean on their sides.

Rosh (Megilah 3:4): It seems that the Rif brought these two teachings together because he holds that Abaye and Rava leaned on their sides due to R. Elazar's teaching. This is wrong. Rather, even though the Torah forbids prostrating on the face only with spreading hands and feet, mid'Rabanan it is forbidden even without spreading hands and feet. Rav Hai Gaon says that Abaye and Rava leaned so it would not look like they bow. The Yerushalmi says that R. Elazar's law applies only one praying in front of the Tzibur for the Tzibur. There is no concern ifhe prays in his house for the Tzibur. If he does so b'Tzibur, it will be shameful for him if people say that he was not worthy to be answered.

Rambam (Hilchos Avodah Zarah 6:8): Anyone who prostrates to Hash-m on split stones without spreading hands and feet, he is lashed mid'Rabanan.

Rambam (Hilchos Tefilah 5:14): When doing Nefilas Apayim after Tefilah, some do Kidah and some prostrate. One may not prostrate on stone except for in the Beis ha'Mikdash, like we explained in Hilchos Avodah Zarah. An important person may not fall on his face unless he knows that he is a Tzadik like Yehoshua. Rather, he tilts his face a little and does not bury it in the ground. One may pray here and fall on his face elsewhere.

Ra'avad: The Yerushalmi says that this is only an individual for the Tzibur. Some texts say 'just an individual b'Tzibur falls on his face.'

Kesef Mishneh: In Hilchos Avodah Zarah, the Rambam explained that Abaye and Rava tilted, lest they be Mishtachaveh on the floor. We can say that the Rambam holds that since the Gemara brought the case of Abaye and Rava after R. Elazar's law, we can say that it applies to both laws. Also, when there was a (stone) floor in front of them, they would tilt even if they were not praying for the Tzibur, lest they prostrate on the floor. Alternatively, even though Abaye and Rava tilted lest they prostrate on the floor, the Gemara connotes that this is not included in (the Isur of) an important person falling on his face.

POSKIM

Shulchan Aruch (OC 131:8): An important person may not fall on his face when he prays for the Tzibur unless he is sure that he will be answered, like Yehoshua.

Pri Chodosh: The Gemara says that this is why Rav did not fall on his face, and Rashi and others explain that this is why Abaye and Rava did not fall on their faces. The Gemara did not say that they were praying for the Tzibur! To resolve the Bavli with the Yerushalmi, we must say that 'Yachid Al ha'Tzibur' means in front of a Tzibur, even if it is not for the Tzibur, unlike the Shulchan Aruch.

Kaf ha'Chayim (108): The Rosh, Tur, Levush and Taz hold like the Shulchan Aruch. They are correct.

Taz (13): This is when the rest of the Tzibur does not fall on their faces, e.g. they do not say Tachanun.

Mishnah Berurah (38): If the entire Tzibur falls on their faces and Chas v'Shalom they are not answered, it is not such a disgrace for the important person, for he is no better than the entire Tzibur.

Magen Avraham (19): If he leans a little, it is permitted.

Mishnah Berurah (37): An important person is forbidden Derech Kidah, even without spreading hands and feet, even if there is not a stone floor. Others are permitted. If he leans a little on his side, it is permitted even on a stone floor. All the more so, our Nefilas Apayim, which is merely tilted the head and covering the face, is permitted.

Tur: Rav Natrunai Gaon says that one who falls on his face must suspend his face above the ground, lest it seem that he prostrates to what is in front of him. It says also in Megilah that Abaye and Rava tilted on their sides, and did not prostrate on their faces, even without spreading hands and feet, even though the Torah forbids only on a (stone) floor with spreading hands and feet, mid'Rabanan it is forbidden even without spreading hands and feet. Rav Amram Gaon says that the Tzibur falls on their faces.

Beis Yosef (DH u'Mah she'Chosav Rabeinu b'Shem): It seems that the Gemara in Megilah is not related to Rav Natrunai Gaon's words. It seems that the Tur holds that Rav Natrunai requires suspending the face above the ground lest it be flush on the floor, lest it seem that he prostrates. Therefore, he brings a proof from Megilah. Rav Natrunai Gaon says that an important person leans to his left. This is unrelated to what he wrote before about the Isur Hishtachava'ah on the ground. Perhaps he does not discuss Hishtachava'ah on the ground. Rather, he teaches that the enactment of Nefilas Apayim is to suspend his face above the ground, that it should not be flush on the floor. Then he teaches that an important person leans to his left, and others may lean to either side. The Tur teaches that we learn from Megilah another matter relevant to Nefilas Apayim, that one may not prostrate on the ground. It is better to say that the Tur explains that Rav Natrunai Gaon teaches two matters. One must suspend his face above the ground, lest it seem that he prostrates on the ground. Also, an important person leans to his left, so even if his face is flush on the floor, it will not look like he prostrates. However, if so why did Rav Natrunai Gaon specify the left side? Also if he leans to his right, it will not look like he prostrates on the ground! The Kol Bo says in the name of Rabbeinu Meir that one does not put his face flush on the floor due to Even Maskis, and an important person leans to his left and lifts his right, for this is how they threw the Korban Tamid to the ground. Also, the one who slaughters it put it on his left.

Beis Yosef (DH u'Mah she'Chosav Rabeinu uv'Ashkenaz): The Tur and Roke'ach say that the custom in Eretz Ashkenaz is to lean on the right side. This is based on Medrashim, that the Shechinah is in front of him, and when he leans to his right, "His left hand is under my head, and His right hand hugs me." Also the Rivash (412) says so in the name of Rashi. Shibolei ha'Leket says in the name of Rav Hai Gaon that one must lean to the left, for leaning to the right is not called reclining. Free people and kings recline to the left. One must be humble in front of Hash-m like a son of the king. R. Binyamin explains that when one prays, the Shechinah is on his right - "Hash-m Tzilcha Al Yad Yeminecha." If one would lean to his right, he would face away from the Shechinah. A slave may not do so to his master! Also the Avudraham says so. This is the custom.