Musnad Ahmad 1153:1115

Muhammad ibn Isma'il ibn Abi Fudayk al-Dayli narrated to us, Ibrahim ibn Isma'il ibn Abi Habibah al-Ash-hali informed us, from Dawud ibn al-Husayn, from al-Qasim ibn Muhammad, from 'Aishah, that 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 seeking the pleasure of the Lord. And in the black seed is a cure for every disease except the sam." They said: "O Messenger of Allah, and what is the sam?" He said: "Death."