Musnad Abu Ya'la al-Mawsili 42:418

And from Abu Sa'id al-Khudri, he said: It was said, "O Messenger of Allah, a day the measure of which is fifty thousand years—how long this is!" The Messenger of Allah, may Allah bless him and grant him peace, said: "By the One in Whose hand is the soul of Muhammad, it will be lightened for the believer until it is lighter upon him than an obligatory prayer that he performs in the world."