Should I not be a thankful servant?

The Prophet ﷺ prayed at night until his feet swelled, out of gratitude.

Narrated by al-Mughirah ibn Shu'bah

The Prophet ﷺ used to stand in prayer at night until his feet swelled. It was said to him: "Allah has forgiven you your past and future sins." He said: "Should I not be a thankful servant?"

Sahih al-Bukhari 1130, Sahih Muslim 2819

What this hadith teaches

Prayer is the language of gratitude. Even the one already forgiven prayed long out of love and thanks.

Reference and grade

Grade
Sahih: agreed upon by al-Bukhari and Muslim
Narrator
al-Mughirah ibn Shu'bah

The hadith in Arabic نص الحديث

لِيُصَلِّيَ حَتَّى تَرِمُ قَدَمَاهُ أَوْ سَاقَاهُ، فَيُقَالُ لَهُ فَيَقُولُ ‏"‏ أَفَلاَ أَكُونُ عَبْدًا شَكُورًا

Arabic text of Sahih al-Bukhari 1130, as given on sunnah.com.