Jami at-Tirmidhi 3522 (sahih, al-Albani)

قُلْتُ لأُمِّ سَلَمَةَ يَا أُمَّ الْمُؤْمِنِينَ مَا كَانَ أَكْثَرُ دُعَاءِ رَسُولِ اللَّهِ صلى الله عليه وسلم إِذَا كَانَ عِنْدَكِ قَالَتْ كَانَ أَكْثَرُ دُعَائِهِ ‏"‏ يَا مُقَلِّبَ الْقُلُوبِ ثَبِّتْ قَلْبِي عَلَى دِينِكَ ‏"‏ ‏.‏ قَالَتْ قُلْتُ يَا رَسُولَ اللَّهِ مَا لأَكْثَرِ دُعَائِكَ يَا مُقَلِّبَ الْقُلُوبِ ثَبِّتْ قَلْبِي عَلَى دِينِكَ قَالَ ‏"‏ يَا أُمَّ سَلَمَةَ إِنَّهُ لَيْسَ آدَمِيٌّ إِلاَّ وَقَلْبُهُ بَيْنَ أُصْبُعَيْنِ مِنْ أَصَابِعِ اللَّهِ فَمَنْ شَاءَ أَقَامَ وَمَنْ شَاءَ أَزَاغَ ‏"‏ ‏.‏

“Shahr bin Hawshab said:“I said to Umm Salamah: ‘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: “O Changer of the hearts, make my heart firm upon Your religion (Yā Muqallibal-qulūb, thabbit qalbī `alā dīnik).’” She said: ‘So I said: “O Messenger of Allah, why do you supplicate so frequently: ‘O Changer of the hearts, make my heart firm upon Your religion.’ 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.’””