Hadith: O Messenger of Allah, may my father and mother ransom you, what is it you say between the (opening) Takbīr and the recitation (of the Qur'an)? He said: I say: O Allah, keep me away from my sins as You have kept the east away from the west. O Allah, cleanse me from my sins as a white garment is cleansed from dirt. O Allah, wash my sins away with water, snow and hail

عَنْ أَبِي هُرَيْرَةَ رضي الله عنه قَالَ:

كَانَ رَسُولُ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ، إِذَا كَبَّرَ فِي الصَّلَاةِ، سَكَتَ هُنَيَّةً قَبْلَ أَنْ يَقْرَأَ، فَقُلْتُ: يَا رَسُولَ اللهِ بِأَبِي أَنْتَ وَأُمِّي أَرَأَيْتَ سُكُوتَكَ بَيْنَ التَّكْبِيرِ وَالْقِرَاءَةِ، مَا تَقُولُ؟ قَالَ «أَقُولُ: اللهُمَّ بَاعِدْ بَيْنِي وَبَيْنَ خَطَايَايَ كَمَا بَاعَدْتَ بَيْنَ الْمَشْرِقِ وَالْمَغْرِبِ، اللهُمَّ نَقِّنِي مِنْ خَطَايَايَ كَمَا يُنَقَّى الثَّوْبُ الْأَبْيَضُ مِنَ الدَّنَسِ، اللهُمَّ اغْسِلْنِي مِنْ خَطَايَايَ بِالثَّلْجِ وَالْمَاءِ وَالْبَرَدِ».
[صحيح] - [متفق عليه]

Abu Hurayrah (may Allah be pleased with him) reported: The Messenger of Allah (may Allah's peace and blessings be upon him) used to stay silent for a little while (once he starts the prayer) before he begins the recitation. I said: "O Messenger of Allah, may my father and mother ransom you, what is it you say between the (opening) Takbīr and the recitation (of the Qur’an)?" He said: "I say: O Allah, keep me away from my sins as You have kept the east away from the west. O Allah, cleanse me from my sins as a white garment is cleansed from dirt. O Allah, wash my sins away with water, snow and hail."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

This supplication suits best such a noble situation, in which one converses privately with Allah, the Almighty, asking Him to erase his sins and to keep him far from them, just as the east and west are kept far and never meet. He also begs Allah to remove his sins and misdeeds and cleanse him therefrom, just as filth is removed from a white garment, and to wash his sins away, and cool their heat and flames with these cold purifiers: water, snow and hail. These are very typical similes. Starting with this supplication cleanses one from the effects of sins, thus one stands before Allah, the Almighty, in the best condition. It is better to recite all the authentic opening supplications alternately. However, this Hadīth of Abu Hurayrah is the most authentic, and Allah knows best.