Sayyiduna al-Mughirah bin Shu'bah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) would stand in prayer until his (peace and blessings be upon him) feet became swollen. It was said to him (peace and blessings be upon him): "Has Allah not forgiven you (peace and blessings be upon him) your former and latter sins?" He (peace and blessings be upon him) said: "Should I not be a grateful servant of Allah?"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4731