Musnad Ahmad 26:686

Yahya narrated to us, from Sufyan, 'Ata' ibn al-Sa'ib narrated to us, from Abu 'Abd al-Rahman al-Sulami, from 'Abdullah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, Allah the Almighty and Majestic did not send down a disease except that He sent down for it a cure; those who know it know it, and those who are ignorant of it are ignorant of it."