Whenever the Messenger of Allah ﷺ said the opening takbir in prayer, he would pause silently for a brief moment. I asked him, "Messenger of Allah, may my father and mother be ransom for you, what do you say during your silence between the takbir and the recitation?"He replied, "I say, 'O Allah, distance me from my sins as You have distanced the East from the West. O Allah, cleanse me of my sins as a white garment is cleansed of filth. O Allah, wash away my sins with snow, water, and hail.'"
حدثنا جرير عن عمارة بن القعقاع عن أبي زرعة
كان رسول الله ﷺ إذا كبر في الصلاة سكت هنية فقلت له يا رسول الله بأبي أنت وأمي ما تقول في سكوتك بين التكبير والقراءة قال أقول اللهم باعد بيني وبين خطاياي كما باعدت بين المشرق والمغرب اللهم أنقني من خطاياي كما ينقى الثوب الأبيض من الدنس اللهم اغسلني من خطاياي بالثلج والماء والبرد
Its isnad is sahih. Jareer is Ibn Abdul-Hameed bin Qurt Dabbi, trustworthy.It is found in Bukhari 2/227, no. 744 (Fath al-Bari) in the Book of adhan, chapter on what is said at the takbir, and in Muslim 1/419, no. 598, in the Book of Mosques, chapter on what is said between the takbir of ihram and the recitation. It is also in Abu Dawud 781, in the Book of Prayer, chapter on the pause after the opening, in Nasai in the Book of Purification, chapter on wudu with snow, and in Ibn Majah 1/264, no. 805.