Musnad Abu Ya'la al-Mawsili 155:241

On the authority of Abu Hurairah, he said: When the Messenger of Allah (peace and blessings of Allah be upon him) would say the Takbir in prayer, he would remain silent for a brief moment before reciting the Quran. I said: "O Messenger of Allah, may my father and mother be sacrificed for you, what do you say during your silence between the Takbir and the recitation?" He said: "I say: 'O Allah, distance me from my sins as You have distanced the East from the West. O Allah, purify me from my sins as a white garment is purified from filth. O Allah, wash away my sins with water, snow, and hail.'"