Meaning: 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.'
Reference: Tirmidhi Vol 4, Book 32, Hadith #2290