Sunan An-Nasa'i

It was narrated from Zahdam that:Some chicken was brought to Abu Musa and a man moved away from the people. He said: ""What is the matter with you?"" He said: ""I saw it eating something that I consider filthy, and I swore I would not eat it."" Abu Musa said: ""Come and eat, for I saw the Messenger of Allah eating it."" And he told him to offer an expiation for his vow (Kafarat Al-Yamin)