A'isha (may Allah swt please with her) reported that the Prophet (may Allah swt shower peace and blessings on him) used to pray at night until his feet became cracked. I said to him, 'Why do you do this, messenger of Allah, when Allah has forgiven you all your past and future wrong actions?' He said, 'Do I not want to be a grateful slave?'

Riyad-us-Sauleheen