'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]. (ES)
App reference (ES): Book 9, Hadith 170 (ES)
Arabic/English book reference (ES): Book 9, Hadith 1160 (ES)