1 - the belief of the heart: its assent and affirmation.
2 - the deeds of the heart: its submission and will, and the deeds of the heart that follow from these, such as reliance upon Allah, hope, fear, and love.
3 - the affirmation of the tongue.
4 - the deeds of the bodily faculties—including the tongue. Deeds encompass both doing and refraining, whether in speech or physical action.

Allah Most High says:
“O you who have believed, believe in Allah and His Messenger and the Book that He sent down upon His Messenger and the Scripture which He sent down before. And whoever disbelieves in Allah, His angels, His Books, His Messengers, and the Last Day has certainly gone far astray.” [Al-Nisā’: 136]

And He Most High says:
“So believe in Allah and His Messenger and the light which We have sent down. And Allah is fully aware of what you do.” [Al-Taghābun: 8]

And He Most High says:
“The Messenger has believed in what was sent down to him from his Lord, and so have the believers. All have believed in Allah, His angels, His Books, and His Messengers.” [Al-Baqarah: 285]

And He Most High says:
“The believers are only those whose hearts tremble when Allah is mentioned, and whose faith increases when His verses are recited to them, and who put their trust in their Lord; * those who establish prayer and spend from what We have provided for them. * Those are truly the believers. For them are ranks with their Lord