Explanation of the 21st hadith in the Heart Therapy Series:

Umm Salamah was asked: “What was the most frequent du‘a of the Prophet salla Allah alayhi wa sallam when he was with you?” She said: “His most frequent du‘a was ‘O Turner of the hearts, make my heart firm upon your religion.’” She said: “O Messenger of Allah, why is ‘O Turner of the hearts, make my heart firm upon your religion’ your most frequent du‘a?” He said: ‘O Umm Salamah, there is no human except that their heart is between Two Fingers of the Fingers of Allah, so whomever He wills He makes steadfast, and whomever He wills He leads astray.’” Then Muadh [one of the narrators of the hadith] recited: (O Our Rabb, do not lead our hearts astray after you have guided us [Aal Imran (8)])
Reported by al-Tirmidhi (3522) who declared it to be hasan and al-Albani declared it to be sahih (Sahih al-Tirmidhi 3522)
قُلتُ لأمِّ سَلَمَةَ يا أمَّ المؤمنينَ ما كانَ أكْثرُ دعاءِ رسولِ اللَّهِ صلَّى اللَّهُ علَيهِ وسلَّمَ إذا كانَ عندَكِ قالَت كانَ أَكْثرُ دعائِهِ: (يا مُقلِّبَ القُلوبِ ثَبِّتْ قلبي على دينِكَ) قالَت فقُلتُ يا رسولَ اللَّهِ ما لأكثرِ دُعاءكَ يا مُقلِّبَ القلوبِ ثبِّت قلبي على دينِكَ قالَ (يا أُمَّ سلمةَ إنَّهُ لَيسَ آدميٌّ إلَّا وقلبُهُ بينَ أُصْبُعَيْنِ من أَصابعِ اللَّهِ فمَن شاءَ أقامَ ومن شاءَ أزاغَ) فتلا معاذٌ [أحد رواة الحديث] رَبَّنَا لَا تُزِغْ قُلُوبَنَا بَعْدَ إِذْ هَدَيْتَنَا [آل عمران (8)].
رواه الترمذي (3522) وحسنه وصححه الألباني (صحيح الترمذي 3522)