Hadith 4379
Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates: Sayyiduna Umar (may Allah be pleased with him) had made a vow during the days of ignorance that he would perform i'tikaf for one night in Masjid al-Haram. So the Prophet Muhammad (peace and blessings be upon him) said to him: Fulfill your vow.