Shahr bin Hawshab (R) said: “I said to Umm Salamah (RA): ‘O Mother of the Believers! What was the supplication that the Messenger of Allah (ﷺ) said most frequently when he was with you?” She said: ‘The supplication he said most frequently was:

يَا مُقَلِّبَ الْقُلُوبِ ثَبِّتْ قَلْبِي عَلَى دِيْنِكَ

Ó Měniči srdcí! Upevni mé srdce ve Tvém náboženství.

Reference: Sahíh. Tirmizí: 3522