Musnad Ahmad 26:372

Mu'ammal narrated to us, Sufyan narrated to us, from 'Ata'—meaning Ibn al-Sa'ib—from Abu 'Abd al-Rahman, from 'Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Allah the Almighty has not sent down any disease, except that He has sent down for it a cure; those who know it know it, and those who are ignorant of it are ignorant of it."