Quotation for Bava Kamma 173:2

It is similar to a slave or maidservant. Rashi explained that the exemption is taught by a Mishna later in Perek Hachovail that says that when a slave damages, his owner is not liable. Tosafot holds that the Mishna Rashi quotes is not the proper source of this ruling.
The source for this ruling in the Mishna is not as Rashi explained because we are taught in a Mishna in perek Hachovail (later 87a) that as far as a slave and a married woman are concerned, an encounter with them is terrible, for one must pay if he injures them and they do not have to pay if they injure anybody. For if it was so that our Gemara is referring to that Mishna, the Gemara should have mentioned ‘a slave and a married woman’, which would be a direct quotation of that Mishna. Since the Gemara does not mention ‘a slave and a married woman’, it seems that the Gemara is not referring to that Mishna.
Rather, Rabbeinu Tam explains that the Gemara mentioned a slave and a slave woman, because it is referring to a Mishna at the end of Masechet Yadayim (Chapter 4, Mishna 7) that specifically discusses this case of a slave and a slave woman who damage others. And the reason that they are exempt which is because their master may anger them and they will damage another person’s property in order to cause their master to pay that is soon mentioned in our Gemara is explicitly mentioned there in Masechet Yadayim, which also proves that our Gemara is referring to that Mishna.