Reference : Hisn al-Muslim 27

اللّهُـمَّ باعِـدْ بَيـني وَبَيْنَ خَطـايايَ
كَما باعَدْتَ بَيْنَ المَشْرِقِ وَالمَغْرِبْ ،
اللّهُـمَّ نَقِّنـي مِنْ خَطايايَ
كَمـا يُـنَقَّى الثَّـوْبُ الأَبْيَضُ مِنَ الدَّنَسْ ،
اللّهُـمَّ اغْسِلْنـي مِنْ خَطايـايَ
بِالثَّلـجِ وَالمـاءِ وَالْبَرَدْ.
Allāhumma bā'id baynī wa bayna khatāyāya
kamā bāa'adta bayn al-mashriqi wal-maghribi,
Allāhumma naqqinī min khatāyāya 
kamā yunaqqa 'th-thawbu 'l-'abyaḍu min ad-danasi, 
Allāhumma 'ghsilnī min khatāyāya, 
bi 'th-thalji wal-mā'i wal-barad.

O Allah, separate me from my sins 
as You have separated the East from the West. 
O Allah, cleanse me of my transgressions 
as the white garment is cleansed of stains. 
O Allah, wash away my sins 
with ice and water and frost.

Reference:
Al-Bukhari 1/181, Muslim 1/419.

Chapter: At the start of the prayer (after takbeer)