Sahih Al Bukhari – Book of Beginning Of Creation

Volumn 004, Book 054, Hadith Number 452.
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Narated By Abu Huraira : The Prophet said, "As long as any-one of you is waiting for the prayer, he is considered to be praying actually, and the angels say, 'O Allah! Be merciful to him and forgive him', (and go on saying so) unless he leaves his place of praying or passes wind (i.e. breaks his ablution)."