Listen to the Excellent Explanation of this important supplication by Dr. Saleh as Saleh (rahimahullaah) at the link : https://abdurrahman.org/2017/07/14/opening-dua-in-prayer-o-allah-separate-me-from-my-sins-as-you-have-separated-the-east-from-the-west/

Narrated Abu Huraira: Allah’s Messenger ﷺ used to keep silent between the Takbir and the recitation of Qur’an and that interval of silence used to be a short one. I said to the Prophetﷺ  “May my parents be sacrificed for you! What do you say in the pause between Takbir and recitation?” The Prophet ﷺ said, “I say,

اللَّهُمَّ بَاعِدْ بَيْنِي وَبَيْنَ خَطَايَايَ كَمَا بَاعَدْتَ بَيْنَ الْمَشْرِقِ وَالْمَغْرِبِ، اللَّهُمَّ نَقِّنِي مِنْ خَطَايَايَ، كَمَا يُنَقَّى الثَّوْبُ الْأَ بْيَضُ مِنَ الدَّنَسِ، اللَّهُمَّ اغْسِلْنِي مِنْ خَطَايَايَ بِالثَّلْجِ وَالْمَاءِ وَالْبَرَدِ

Allaahumma baa’id baynee wa bayna khataayaaya kamaa baa’adta baynal-mashriqi walmaghribi, Allaahumma naqqinee min khataayaaya kamaa yunaqqath-thawbul-‘abyadhu minad-danasi, Allaahum-maghsilnee min khataayaaya, bith-thalji walmaa’i walbarad

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.

Al-Bukhari 1/181, Muslim 1/419

Translation Source: From the Book “Fortress of the Muslim” ,
Dr. Sa’eed bin Ali Al-Qahtani, Dar-us-Salam Publication