The Messenger of Allah ﷺ stood in prayer until his feet became swollen. It was said to him, "Messenger of Allah, Allah has forgiven you your past and future sins."He replied, "Should I not, then, be a grateful servant?"
حدثنا سفيان عن زياد بن علاقة سمع
قام رسول الله ﷺ حتى تورمت قدماه فقيل له يا رسول الله قد غفر الله لك ما تقدم من ذنبك فقال أولا أكون عبدا شكورا
Its isnad is sahih. Bukhari related the hadith in Fath al-Bari 2/63, in the chapter on tahajjud, on the Prophet's ﷺ standing in prayer, and Muslim in 4/2171, number 2819, and Tirmidhi in 2/268, number 412, where he said, hasan sahih, and Nasai in 3/219, number 1644, and Ibn Majah in 1/456, number 1419.