Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates that the Messenger of Allah (peace and blessings be upon him) used to observe i'tikaf in the last ten (days) of Ramadan. Nafi' said: Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) showed me the place in the mosque where he (peace and blessings be upon him) used to observe i'tikaf.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8569