Musnad Ahmad 1017:112

Yahya ibn Sa'id narrated to us, from al-Muthanna ibn Sa'id, and Abu Dawud narrated to us, al-Muthanna ibn Sa'id—meaning al-Dab'i—narrated to us, from Qatadah, from Abdullah ibn Buraydah, from his father, that he visited a brother of his and saw his forehead sweating. He said: "Allahu Akbar! I heard the Messenger of Allah (peace and blessings of Allah be upon him) say"—or the Messenger of Allah (peace and blessings of Allah be upon him) said (Abu Dawud said in his narration: "I heard the Messenger of Allah (peace and blessings of Allah be upon him) say"): "The believer dies with sweat on his forehead."