Mustadrak al-Hakim — Hadith 1947

It is narrated from Sayyiduna al-Nawwas bin Sam'an al-Kilabi (may Allah be pleased with him) that he said: I heard the Messenger of Allah (peace and blessings be upon him) say: "There is no heart except that it is between two of the fingers of the Most Merciful; if He wishes, He keeps it upright, and if He wishes, He makes it deviate (go astray)." And the Messenger of Allah (peace and blessings be upon him) used to say: 'Allahumma ya muqallibal-qulub, thabbit qalbi 'ala dinik' (O Turner of hearts, make my heart firm upon Your religion). And the Balance is in the Hand of the Most Merciful; He raises some people and lowers others until the Day of Resurrection.
This hadith is sahih (authentic) on the condition of Imam Muslim, but the two Shaykhs (Bukhari and Muslim) did not record it.