Nafi' narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) said: I vowed to observe i'tikaf in al-Masjid al-Haram. When I accepted Islam, I asked the Prophet (peace and blessings be upon him) about it, and he (peace and blessings be upon him) said: "Fulfil your vow."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النذور / 20099
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