Shahr bin Hawshab said:
“I said to Umm Salamah (wife of Prophet Muhammad ﷺ): ‘ O Mother of the Believers! What was the supplication that the Messenger of Allah (ﷺ) said most frequently when he was with you?”
She said: ‘The supplication he said most frequently was: “O Changer of the hearts, make my heart firm upon Your religion (Yā Muqallibal-qulūb, thabbit qalbī `alā dīnik).’”
She said: ‘So I said: “O Messenger of Allah, why do you supplicate so frequently: ‘O Changer of the hearts, make my heart firm upon Your religion.’ He said: ‘ O Umm Salamah! Verily, there is no human being except that his heart is between Two Fingers of the Fingers of Allah, so whomsoever He wills He makes steadfast, and whomever He wills He causes to deviate.’”
– Authentic Hadith in Sunan At-Tirmithi. Hadith #3522