Musnad Abu Ya'la al-Mawsili 42:264

Zuhayr narrated to us, Abu 'Amir al-'Aqadi narrated to us, from Zuhayr ibn Muhammad, from Muhammad ibn 'Amr, from 'Ata' ibn Yasar, from Abu Hurayrah and Abu Sa'id, from the Prophet (may Allah bless him and grant him peace), who said: "No fatigue, nor disease, nor anxiety, nor sadness, nor grief, nor harm befalls the believing person—even a thorn that pricks him—but that Allah expiates some of his sins thereby."