Hadith: The Prophet (may Allah's peace and blessings be upon him) used to stand in prayer at night until the skin of his feet would crack

عن عائشة رضي الله عنها والمغيرة بن شعبة رضي الله عنه : أن النبي صلى الله عليه وسلم كان يقوم من الليل حتى تَتَفَطَّرَ قَدَمَاهُ فقلت له: لم تَصْنَعُ هذا يا رسول الله، وقد غفر اللهُ لك ما تقدم من ذنبك وما تأخر؟ قال: «أَفَلَا أحب أن أكونَ عبدا شَكُورًا».
[صحيح] - [حديث عائشة -رضي الله عنها-: متفق عليه. حديث المغيرة بن شعبة -رضي الله عنه-: متفق عليه]

‘Ā'ishah and Al-Mughīrah ibn Shu‘bah (may Allah be pleased with both of them) reported: The Prophet (may Allah's peace and blessings be upon him) used to stand in prayer at night until the skin of his feet would crack. I (‘Ā'ishah) asked him: "O Messenger of Allah, why are you doing this while your past and future sins have been forgiven for you?" He said: "Should I not like to be a grateful slave (of Allah)?"
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

The Prophet (may Allah's peace and blessings be upon him) used to stand and pray at night until the skin on his feet would crack. ‘Aishah (may Allah be pleased with her) said to him - thinking that he was worshiping Allah out of fear of sins and seeking forgiveness and mercy from Allah, while Allah has already forgiven his sins and he is in no need of that - "Why are you doing this, O Messenger of Allah, while Allah has forgiven your past and future sins?" He replied: "Should I not be a thankful slave." So this worship was an expression of his gratitude towards Allah for His forgiveness.