The evidence that faith is conviction of the heart is His saying: “Faith has not yet entered your hearts” [al-Hujurat 49:14]; and His saying: “He has made faith dear to you and made it pleasing in your hearts” [al-Hujurat 49:7]; and His saying: “He has inscribed faith upon their hearts” [al-Mujadilah 58:22]. The Exalted also says:

“O Messenger, do not let those who hasten into disbelief grieve you—those who say with their mouths, ‘We believe,’ while their hearts have not believed” [al-Ma'idah 5:41].

And the hadith of Abu Barzah, Buraydah, and al-Bara', from the Prophet ﷺ: “O company of those who have professed faith with their tongues, while faith has not yet sincerely reached their hearts…”