It is narrated from Sayyiduna Mughirah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) would pray for so long that his (peace and blessings be upon him) feet would swell. It was said to him (peace and blessings be upon him) about this: "Do you do this when Allah has forgiven you your past and future sins?" He (peace and blessings be upon him) said: "Should I not be a grateful servant?"
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 18135