مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4208
Musnad of Abdullah bin Mas'ood
It was narrated that ‘Abdullah (رضي الله عنه) said: The Messenger of Allah (ﷺ) said: "Allah, may He be glorified and exalted, has not sent down any disease, but He has also sent down a remedy for it. Those who know it know it, and those who do not know it do not know it."
Musnad Ahmad 4236aheeh because of corroborating evidence; its isnad is hasan]