Musnad Ahmad 31:726

Hassan narrated to us, Ibn Lahi'ah narrated to us, Darraj narrated to us, from Abu al-Haytham, from Abu Sa'id al-Khudri, who said: It was said to the Messenger of Allah, peace and blessings of Allah be upon him, regarding the day whose measure is fifty thousand years: "How long is this day?" The Messenger of Allah, peace and blessings of Allah be upon him, said: "By the One in Whose hand is my soul, it is indeed lightened for the believer, until it is lighter upon him than an obligatory prayer he performs in the world."