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 قَلْبِيْ عَلَى دِيْنِكَ

Ya Muqallibal-qulubi, thabbit qalbi 'ala dinik

O Changer of the Hearts! Strengthen my heart upon Your Religion. So I said: O Messenger of Allah (ﷺ), why do you supplicate so frequently. He said: ‘O Umm Salamah! Verily, there is no human being except that his heart is between Two Fingers of the Fingers of Allah, so whomsoever He wills He makes steadfast, and whomever He wills He causes to deviate.

Reference: Sahih (Albani). Tirmidhi: 3522