Sayyiduna Abu Hurayrah (may Allah be pleased with him) says that when the Messenger of Allah (peace and blessings be upon him) said the takbir (tahrimah) for prayer, he would remain silent for a short while before the recitation. I said: May my father and mother be sacrificed for you, what do you recite in the silence you keep between the takbir and the recitation? He (peace and blessings be upon him) said: "I say: 'Allahumma ba'id bayni wa bayna khatayaya kama ba'adta bayna al-mashriqi wal-maghrib. Allahumma naqqini min khatayaya kama yunaqqa al-thawbu al-abyadu min al-danas. Allahumma-ghsilni min khatayaya bil-thalji wal-ma'i wal-barad' (O Allah, put a distance between me and my sins as You have put a distance between the east and the west. O Allah, cleanse me of my sins as a white garment is cleansed of dirt. O Allah, wash away my sins from me with snow, water and hail)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3073