(174) بَابُ ارْحَمْ مَنْ فِي الأَرْضِ
Mu'awiya ibn Qurra reported that his father said, "A man said,'Messenger of Allah, I was going to slaughter a sheep and then I felt sorryfor it (or 'sorry for the sheep I was going to slaughter').' He said twice,'Since you showed mercy to the sheep, Allah will show mercy to you.'"
Reference: Al-Adab Al-Mufrad 373
In-book reference: Book 20, Hadith 2
English translation: Book 20, Hadith 373