The Mishnah spoke about acquiring slaves. R. Simon, brother of R. Yehudah bar Zavdi citing Rav, said - A child, as long as he is abandoned in the market, his father or mother can testify about his identity.

Once he has been taken in from the market, we require two witnesses, and his father and mother act as two witnesses.

Question (R. Abahu): But isn't a person suspected to say that someone who is not his son, is in fact his son (so why are they believed in the market)?

Answer: Perhaps that is only when there is a concern that he is a convert without inheritors, who this 'father' wishes to inherit, but otherwise, we would believe the father.

Question: But all of the Rabbis of the Diaspora testify that they heard from Rav Ada bar Avuhah that Rav Chisda said that he is not believed (at all)!

Answer: They are believed about a child who is too young to walk (so he is assumed to be their child), but if he could walk from place to place, they are not believed.

It is like the statement of R. Yochanan, that there is no law of chazakah for calves and foals who hop around from place to place.