Ahmed bin Suliaman أحمد بن سليمان · The Book of Oaths and Vows

"A man from Banu Ghifar told me, in the gathering of Salim bin 'Abdullah, Salim bin 'Abdullah said: 'I heard 'Abdullah -that is, Ibn 'Umar- say: "The Messenger of Allah said: 'Allah forbids you to swear by your forefathers.'"'"

"Do not swear by your forefathers or by false gods (At-Tawaghit)."

"Umar had made a vow to spend a night in 'Itikaf in Al-Masjid Al-Haram. He asked the Messenger of Allah about that, and he ordered him to perform the 'Itikaf."