Affan bin Muslim عفان بن مسلم بن عبد الله الصفار · The Book of Oaths and Vows

"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."