Ziyad bin Ilaqah (may Allah have mercy on him) narrates: I heard Mughirah bin Shu'bah (may Allah be pleased with him) say: The Prophet (peace and blessings be upon him) would pray for so long that his (peace and blessings be upon him) feet or shins would swell. He (peace and blessings be upon him) would be asked (the reason for it), and he (peace and blessings be upon him) would say: "Should I not be a grateful servant?"
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 18133