Nafi' narrates from Ibn Umar (may Allah be pleased with them both) that Umar (may Allah be pleased with him) said to the Prophet (peace and blessings be upon him): In the Jahiliyyah I vowed to observe i'tikaf in al-Masjid al-Haram. He (peace and blessings be upon him) said: "Fulfil your vow."
(b) In the narration of Musaddad: I vowed in the Jahiliyyah that I would observe i'tikaf for a night.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النذور / 20100