From Sahih Bukhari (Hadith 4320):
Narrated Ibn `Umar:When we returned from (the battle of) Hunain, `Umar asked the Prophet (ﷺ) about a vow which he had made during the Pre-Islamic period of Ignorance that he would perform I`tikaf. The Prophet (ﷺ) ordered him to fulfill his vow
Reference: Bukhari 4320