It is narrated from Sayyiduna al-Mughirah bin Shu'bah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) would pray until his blessed feet became swollen. This was mentioned to him, and he (peace and blessings be upon him) said: "Should I not be a grateful servant of Allah?"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13273