Musannaf Ibn Abi Shayba 7:129

Narrated to us Abd al-A’la, from Hisham, from al-Hasan, regarding a man upon whom is the feeding of poor persons for the expiation of *zihar* (injurious comparison). He fed ten, then he wanted to repeat [the feeding] to them until he completed [the number]. He said: "No, not until he feeds sixty poor persons."