The Book of Prayers (Kitab Al-Salat)

Muslim :: Book 4 : Hadith 1245

Abu Huraira reported that Allah's Messenger (may peace be upon him) used to observe, silence for a short while between the takbir (at the time of opening the prayer) and the recitation of the Qur'an. I said to him: Messenger of Allah, for whom I would give my father and mother in ransom, what do you recite during your period of silence between the takbir and the recitation? He said: I say (these words):" O Allah, remove my sins from me as Thou hast removed the East from the West. O Allah purify me from sins as a white garment is purified from filth. O Allah! wash away my sins with water, snow and hall."

Source materials are from the University of Southern California MSA site
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