Musnad Abu Ya'la al-Mawsili 160:243

Salih ibn Malik narrated to us, Abd al-A’la ibn Abi al-Masawir narrated to us, Muhammad ibn ‘Amr ibn ‘Ata’ narrated to us, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There is no servant who falls ill with an illness except that Allah commands his guardian (angel) that whatever evil deed he committed, do not write it down, and whatever good deed he committed, write down ten good deeds for him. And (He commands) that there be written for him of righteous deeds the same as he used to do while he was healthy, even if he does not perform them."