Mughirah bin Shu'bah Radiyallahu 'Anhu reports that Rasuluilah Sallallahu 'Alayhi Wasallam performed such lengthy nafl prayers, that his mubaarak legs became swollen. The Sahaabah said :
"You undergo such great difficulties, where Allah had forgiven your past and the future sins. "Rasulullah Sallallahu 'Alayhi Wasallam said: "(When Allah Ta'aala has blessed me so much) should I not be a grateful servant?" (FR)
English reference (FR): Book 39, Hadith 248 (FR)
Arabic reference (FR): Book 40, Hadith 261 (FR)