'Amro bin Shu'aib narrated on the authority of his father, on the authority of his grandfather (RAA) that the Messenger of Allah (ﷺ) said:
"Anyone who practices medicine but is not known as a practitioner, and kills a human being or inflicts harm on him, will be held responsible." Related by Ad-Daraqutni. Al-Hakim graded it as Sahih. Abu Dawud, an-Nasa'i and others also narrated it, but its Mursal form is stronger than the connected one.
English reference: Book 9, Hadith 1224
Arabic reference: Book 9, Hadith 1195