Rashi: To teach us that whenever the Torah writes "Eid" without the word "Echad", it means two witnesses, ¹ even though the word "Eid" implies one witness. ²

Sifri: It implies that, although one witness is not believed to testify on behalf of (or against) a man, he is believed to testify on behalf of a woman ? that her hssband died, to authorize her to remarry. ¹

Rashi and Targum Yonasan #1: It implies that, although one witness is not believed to implicate the accused regarding Malkos or Misah or to obligate him to pay, he is believed to obligate him to make a Shevu'ah ¹ where the defendant denies owing the claimant money. ²

Targum Yonasan #2: It implies ³ that, although one witness is not believed to implicate the accused regarding Malkos or to obligate him to pay, he is believed to prevent a Sotah from drinking the water should he testify that she is guilty. ⁴

Sifri: "le'Chol Avon" incorporates Makos; "le'Chol Chatas" - Korbanos; "le'Chol Chet" - elevating a Kohen to the Kehunah or relegating him from the Kehunah; ⁵ "asher Yecheta - "Dinei Mamonos'. ⁶

Sifri: To incorporate in the La'av one witness who testifies on a woman. ¹

Refer to 19:5:2:2 and note 1.

Pesachim, 113b: No! Because the testimony of only one witness achieves nothing ¹ and is considered Motzi-Shem-Ra.

Rashi #1: It implies a. that the witnesses must verbalize their testimony, and not write it on a piece of paper and send it to Beis-Din, ¹ and b. that Beis-Din must hear the testimony from the mouth of the witnesses and not from the mouth of a translator. ²

Rashi #2 (in Yevamos, 117a): It teaches us that a woman is not believed to authorize her husband's heirs to inherit the property of her husband on whose death she is testifying, even though she is believed with regard to herself remarrying. ³

Ritva (in Kesuvos, 22b): It teaches us that the principle 'Shesikah ke'Hoda'ah' (Silence is akin to admission') does not apply to witnesses, and that the witnesses (each witness) are therefore obligated to verbalize their testimony. ⁴

Yerushalmi Yoma, 6:1: It teaches us via a Gezeirah Shavah "Sh'nei" "Ve'amdu Sh'nei ha'Anashim" - in Pasuk 17 - that women and Ketanim are not eligible to judge or to testify. ⁵

Kidushin, 65b: When Mar Zutra and Rav Ada Saba divided their property without witnesses, Rav Ashi told them that their transaction was final and that they were not permitted to retract (without mutual consent) because, with regard to financial transactions, witnesses are only required to prevent one of the partners from subsequently lying.

Rashi in Yevamos, 117a: To teach us that, although a woman who testifies that her husband died is believed regarding herself, she is not believed vis-a-vis her chifren, to allow them to inherit their father's property ? for which two witneses are required. ¹

Yerushalmi Yoma, 6:1: To teaxh us via a Gezeirah Shavah "Sh'nei" "Sh'nei" from Pasuk 17, that a woman and a Katan are not eligible to judge or to testify. ²

Bava Basra, 119b: To teach us that, although two witnesses will suffice for a regular Get, a Get Mekushar requires three. ¹

Makos, 5b #1: To teach us that, just as three witnesses can render two Zom'min, so too can two render three Zom'min. ²

Makos, 5b #2 (citing R. Shimon): To compare three witnesses to two - inasmuch as, just as two witnesses can only be sentenced to death if they have both been declared Zom'min, so too, three witnesses can only be sentenced to death if all three have both been declared Zom'min,

Makos, 5b #3 (citing R. Akiva: To compare three witnesses to two - inasmuch as, just as two witnesses are disqualified if one of them is a relative of the accused, so too, do three ³ - and even a hundred, which we learn from the word "Eidim", which is otherwise superfluous.

Bava Kama, 70b: In order to extrapolate "Davar", 've'Lo Chatzi Davar' - to teach us that if two witnesses ¹ testifythat a girl has one pubic hair on her back and two other witnesses, that she has one hair on her stomach, their testimony is invalid and she is still a Ketanah.

Makos, 6a (according to R. Akiva): Bearing in mind that if one of three witness is Pasul, the entire testimony is disqualified, neither the murderer nor the victim disqualify the testimony, ² since "Yakum Davar" pertains exclusively to the witnesses. ³

Because there are four Pesukum ('Arba'ah Kesuvim ha'Ba'im ke'Echad') which teach us that "Eid" is one witness. Tosfos (Kidushin 35a DH 'Ela') says that Shnei ¹ Kesuvim are like an exclusion; they teach that this does not apply elsewhere. And an exclusion is stronger than a Binyan Av (PF)

The Moshav Zekenim (in Bamidbar 5:13) stated that in "Lo Yakum?", the word "Echad" is superfluous. If 'Eid' would always mean one, the Torah should have omitted it, since there is nothing superfluous in the Pasuk "Eid Chamas" (PF).