Musnad Ahmad 26:784

'Affan narrated to us, Hammam narrated to us, 'Ata' ibn al-Sa'ib informed us, that Abu 'Abd al-Rahman narrated to him, that 'Abdullah ibn Mas'ud 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 with it a cure" — and 'Affan said once: "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."