Ibne Abbas R.A. reports:
"Abu Bakr R.A. once said, 'O Messenger of Allah, you have become old'. (What is the reason? Due to this mild and soft temperament he ought to be young, and also according to his age he could be still called young). Rasulullah (Sallallahu alaihe wasallam) replied: "The recitation of Surahs Hud, Waaqiah, Mursalat, Amma and Kuwwirah has made me old'".
English reference: Book 5, Hadith 40
Arabic reference: Book 5, Hadith 41