It is narrated from Sayyiduna Mughirah bin Shu'bah (may Allah be pleased with him); he said: The Messenger of Allah (peace and blessings be upon him) would pray for so long that his (peace and blessings be upon him) feet would crack. It was said to him (peace and blessings be upon him): "Has Allah not 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 / 18134