Sunan Ibn Majah, Volume 5, Book 34, Hadith 3834 (DarusSalam) ⓘ

It was narrated that Anas bin Malik said: "The Messenger of Allah (saas) often used to say: 'Allahumma thabbit qalbi 'ala dinika [O Allah, make my heart steadfast in (adhering to) Your religion].' A man said: 'O Messenger of Allah! Do you fear for us when we have believed in you and in (the Message) that you have brought?' He said: 'Hearts are between two of the fingers of the Most Merciful, and He controls them.'" (Hasan)Al-A'mash (one of the narrators) indicated with his fingers.

حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ نُمَيْرٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا الأَعْمَشُ، عَنْ يَزِيدَ الرَّقَاشِيِّ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ كَانَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ يُكْثِرُ أَنْ يَقُولَ ‏"‏ اللَّهُمَّ ثَبِّتْ قَلْبِي عَلَى دِينِكَ ‏"‏ ‏.‏ فَقَالَ رَجُلٌ يَا رَسُولَ اللَّهِ تَخَافُ عَلَيْنَا وَقَدْ آمَنَّا بِكَ وَصَدَّقْنَاكَ بِمَا جِئْتَ بِهِ فَقَالَ ‏"‏ إِنَّ الْقُلُوبَ بَيْنَ إِصْبَعَيْنِ مِنْ أَصَابِعِ الرَّحْمَنِ عَزَّ وَجَلَّ يُقَلِّبُهَا ‏"‏ ‏.‏ وَأَشَارَ الأَعْمَشُ بِإِصْبَعَيْهِ ‏.‏