Musnad Ahmad
It was narrated from ʼUmar that he said: O Messenger of Allah (ﷺ), I vowed during the jahiliyyah to observe i’tikaf for one night in al-Masjid al-Haram. He said to him: “Fulfil your vow.”
حَدَّثَنَا يَحْيَى، عَنْ عُبَيْدِ اللَّهِ، حَدَّثَنِي نَافِعٌ، عَنِ ابْنِ عُمَرَ، عَنْ عُمَرَ، رَضِيَ اللَّهُ عَنْهُ أَنَّهُ قَالَ يَا رَسُولَ اللَّهِ إِنِّي نَذَرْتُ فِي الْجَاهِلِيَّةِ أَنْ أَعْتَكِفَ فِي الْمَسْجِدِ الْحَرَامِ لَيْلَةً فَقَالَ لَهُ فَأَوْفِ بِنَذْرِكَ
Grade: Sahih (Darussalam) [ al-Bukhari (2042) and Muslim(1656) (Darussalam)
Reference