From Sahih Bukhari (Hadith 4458):
Narrated `Aisha:We poured medicine in one side of the Prophet's mouth during his illness and he started pointing to us, meaning to say, "Don't pour medicine in my mouth." We said, "(He says so) because a patient dislikes medicines." When he improved and felt a little better, he said, "Didn't I forbid you to pour medicine in my mouth ?" We said, " ( We thought it was because of) the dislike, patients have for medicines. He said, "Let everyone present in the house be given medicine by pouring it in his mouth while I am looking at him, except `Abbas as he has not witnessed you (doing the same to me)
Reference: Bukhari 4458