'Aishah (May Allah be pleased with her) said:
The Prophet (ﷺ) would stand (in prayer) so long that the skin of his feet would crack. I asked him, "Why do you do this while your past and future sins have been forgiven?" He said, "Should I not be a grateful slave of Allah?"[Al-Bukhari and Muslim].
Arabic/English book reference: Book 1, Hadith 98