Musnad Ahmad 1153:2366

'Amir ibn Salih narrated to us, he said: Hisham ibn 'Urwah narrated to me, from his father, from 'Aishah, that the Messenger of Allah, peace and blessings of Allah be upon him, said: "A believer is not afflicted with a thorn, or anything beyond it, except that Allah removes a sin from him by it."