Shamail Tirmidhi

It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him), he says that the Messenger of Allah (peace and blessings be upon him) used to pray so much that both of his blessed feet would become swollen. So it was said to him, "Do you do this (with such hardship) when you have already been given the glad tidings that Allah Almighty has forgiven your past and future sins?" The Prophet (peace and blessings be upon him) replied: "Should I not then be a grateful servant (of Allah Almighty)?"

It is narrated from Sayyiduna Abu Hurairah, may Allah be pleased with him, that he said: The Messenger of Allah, peace and blessings be upon him, would stand in prayer for such a long time that both of his blessed feet would swell. So it was said to him: You do this (with such hardship) even though Allah Almighty has forgiven all your past and future sins? The Prophet, peace and blessings be upon him, replied: “Should I not be a grateful servant?”