As-Salaam alaikum,
Anas Ibn Malik, may Allah be pleased with him, said :-"The Messenger of Allah, Sallallahu alaihi Wasallam, would often say :-" Ya Muqallib-l-qulubi thabbit qalbi ala dinik- O changer of the hearts, make my heart firm upon Your religion ".
So I asked, 'O Messenger of Allah, we believe in you and what you have come with, but do you have fear for us? "
He replied :" Yes: Indeed the hearts are in between the 2 Fingers of Allah's Fingers. He changes them as He wills." (Tirmidhi 3522).

Related to this is the hadith in Sunan Ahmad wherein the Messenger of Allah, Sallallahu alaihi Wasallam, said :" Shaytan comes to one of you and says : 'who created you?' He replies, 'Allah'. Then Shaytan says: 'who created Allah?' If that happens to anyone of you, let him say 'Amantu bi-llahi wa Rasullih' - I believe in Allah and His Messengers. '. Indeed it will go away from him. "(Ahmad, 25,671).