Musannaf Ibn Abi Shayba 6:16

Ja'far ibn 'Awn narrated to us, he said: Hisham ibn Sa'd narrated to us, he said: I heard 'Urwah ibn Ruwaym mentioning from al-Qasim, from Mu'adh, who said: When Allah afflicts a servant with illness, He says to his companion on the left: "Lift [your pen]." And He says to his companion on the right: "Write for My servant what he used to do."