Musnad Abu Ya'la al-Mawsili 150:65

Muhammad ibn Abdullah ibn Numayr narrated to us, Humayd ibn Abd al-Rahman narrated to us, from Ibrahim ibn Isma'il, from Dawud ibn al-Husayn, from al-Qasim ibn Muhammad, from Aisha, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The siwak is a purification for the mouth and a means of pleasure for the Lord." And in the black seed is a cure for every disease except the sam. I said: "And what is the sam?" He said: "Death."