Maliks Muwatta Book 56, Hadith Number 7.

Malik related to me from Zayd ibn Aslam that Abdullah ibn Umar said, “Two men from the east stood up and spoke, and people were amazed at their eloquence. The Messenger of Allah, may Allah bless him and grant him peace, said, ‘Some eloquence is sorcery,’ or he said, ‘Part of eloquence is sorcery.'”