'Anas narrated:
"The Messenger of Allah (s.a.w) would often say: Ya Muqallibal-qulub, thabbit qalbi 'ala dinik('O Changer of the Hearts! Strengthen my heart upon Your Religion.)' So I said: 'O Prophet of Allah! We believe in you and what you have come with, but do you fear for us?' He said:'Yes. Indeed the hearts are between two Fingers of Allah's Fingers, He changes them as He wills.'" (FR)
English reference (FR): Vol. 4, Book 6, Hadith 2140 (FR)
Arabic reference (FR): Book 32, Hadith 2290 (FR)