Hadith: The Messenger of Allah (may Allah's peace and blessings be upon him) would often say: "O Turner of the hearts, make my heart firm upon Your religion

عَنْ أَنَسٍ رضي الله عنه قَالَ:
كَانَ رَسُولُ اللهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يُكْثِرُ أَنْ يَقُولَ: «يَا مُقَلِّبَ القُلُوبِ ثَبِّتْ قَلْبِي عَلَى دِينِكَ»، فَقُلْتُ: يَا رَسُولَ اللهِ، آمَنَّا بِكَ وَبِمَا جِئْتَ بِهِ فَهَلْ تَخَافُ عَلَيْنَا؟ قَالَ: «نَعَمْ، إِنَّ القُلُوبَ بَيْنَ أُصْبُعَيْنِ مِنْ أَصَابِعِ اللهِ يُقَلِّبُهَا كَيْفَ يَشَاءُ».
[صحيح] - [رواه الترمذي وأحمد] - [سنن الترمذي: 2140]

Anas (may Allah be pleased with him) reported:
The Messenger of Allah (may Allah's peace and blessings be upon him) would often say: "O Turner of the hearts, make my heart firm upon Your religion." I said: "O Messenger of Allah, we have believed in you and what you have come with. Do you fear for us?" He said: "Yes! Indeed, the hearts are between two of the Fingers of Allah and He turns them as He wishes."
[Authentic hadith] - [Narrated by At-Termedhy & Ahmad] - [Sunan At-Termedhy - 2140]

Explanation

The supplication the Prophet (may Allah's peace and blessings be upon him) used to say most frequently is to ask Allah for steadfastness to the religion and obedience to Him and keeping away from deviation and misguidance. Anas ibn Mālik (may Allah be pleased with him) wondered at the Prophet (may Allah's peace and blessings be upon him) making this supplication frequently. So, the Prophet (may Allah's peace and blessings be upon him) informed him that the hearts lie between two of the Fingers of Allah and He turns them as He wishes. The heart is the place where Imān or Kufr lies. It is called Qalb (in Arabic) due to its frequent Taqallub (fluctuation). It fluctuates more intensely than a boiling jar. So, Allah makes the heart of whom He wills steadfast to guidance and firm upon the religion, and He diverts the heart of whom He wills from guidance to deviation and misguidance.