It was narrated from Salim bin' Abdullah bin 'Umar, from his father, from 'Umar, that the Messenger of Allah P.B.U.H heard him swearing by his father. The Messenger of Allah P.B.U.H said: "Allah forbids you from making oaths by your forefathers." 'Umar said: "I never took an oath by them (i.e., my forefathers) myself nor narrating such words from anyone else." (Sahih)