Musnad Ahmad 28:349

Waki’ and Ishaq—meaning al-Azraq—narrated to us, they said: Sufyan narrated to us, from Alqamah ibn Marthad, from al-Qasim ibn Mukhaymirah, from Abdullah ibn Amr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There is no one among the Muslims who is afflicted with a trial in his body, except that Allah—the Almighty and Majestic—commands the guardian angels who protect him: 'Write for My servant the like of what he used to do while he was healthy, as long as he is confined in My bonds.'" Abdullah [ibn Ahmad] said: My father said, and Ishaq said: "Write for My servant in every day and night."