Musnad Ahmad 1017:88

Bahz narrated to us, Muthanna bin Sa'id narrated to us, from Qatadah, from Ibn Buraydah, from his father, that he was in Khurasan. He visited a brother of his who was ill and found him at the point of death, and his forehead was sweating. He said: "Allahu Akbar (God is the Greatest). I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'The death of the believer is by the sweat of the forehead.'"