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

Status 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.

English Translation:

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.