Musnad Ahmad 626:0

Narrated to us by Abd al-Rahman, from Sufyan, from Mansur, from al-Mundhir ibn Jarir, from his father, from al-Mughirah ibn Shu’bah, who said: The Prophet (ﷺ) stood until his feet—or his shins—swelled. It was said to him, "You do this, while Allah has forgiven you your past and future sins?" He replied, "Shall I not be a thankful servant?"