The Prophet ﷺ prayed until his feet became swollen. He was asked, "Why do you burden yourself with this when Allah has already forgiven your past and future sins?"He replied, "Should I not be a grateful servant?"
أن النبي ﷺ صلى حتى انتفخت قدماه، فقيل له: أتكلف هذا، وقد غفر الله لك ما تقدم من ذنبك وما تأخر؟ فقال: أفلا أكون عبدا شكورا.
Bukhari narrated it in tahajjud (1130), and Muslim in the characteristics of the hypocrites (2819), both from the hadith of Ziyad bin Alaqa, from Mughira bin Shuba, who mentioned it.