Musnad Ahmad 31:594

Isma'il narrated to us, Muhammad ibn Ishaq narrated to us, from Muhammad ibn 'Amr ibn 'Ata, from 'Ata ibn Yasar, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The believer is not afflicted by fatigue, nor exhaustion, nor sadness, nor illness, nor harm—not even the anxiety that worries him—but that Allah expiates some of his sins thereby."