Abu Hurayrah Radiyallahu 'Anhu says:
"Rasulullah Sallallhu 'Alayhi Wasallam performed so many nawaafil prayers that his legs swelled. Someone said to him, you take so many pains, whereas you have been given the good news that your past and present sins have been forgiven? He replied: "Should I not be grateful servant".
English reference: Book 39, Hadith 249
Arabic reference: Book 40, Hadith 262