Riyad as-Salihin

'Aishah (May Allah be pleased with her) reported: The Prophet (ﷺ) kept standing (in prayer) so long that the skin of his feet would crack. I asked him: "Why do you do this, while you have been forgiven of your former and latter sins?" He said, "Should I not be a grateful slave of Allah?"

[Al-Bukhari and Muslim].