Sahih Bukhari Volume 001, Book 008, Hadith Number 436.
Narated By Abu Huraira : Allah's Apostle said, "The angels keep on asking Allah's forgiveness for anyone of you, as long as he is at his Mu,salla (praying place) and he does not pass wind (Hadath). They say, 'O Allah! Forgive him, O Allah! be Merciful to him."