Musnad Ahmad 31:200

Yahya narrated to us, from Usamah, who said: Muhammad ibn ‘Amr ibn ‘Ata’ narrated to me, from ‘Ata’ ibn Yasar, from Abu Sa‘id al-Khudri, from the Prophet, peace and blessings of Allah be upon him, who said: "No illness, nor fatigue, nor grief, nor even anxiety that concerns a Muslim befalls him, but that Allah, the Almighty and Majestic, expiates some of his sins because of it."