Maliks Muwatta Book 22, Hadith Number 14.

Section : Oaths in General.

Yahya related to me from Malik from Nafi from Abdullah ibn Umar that one time the Messenger of Allah, may Allah bless him and grant him peace, was speaking to Umar ibn al-Khattab while he was travelling with a troop and Umar swore by his father and he (the Messenger) said, “Allah forbids you to swear by your fathers. If anyone swears, let him swear by Allah or keep silent.”