Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2637
Abdullah b. Umar (Allah be pleased with both of them) reported that the Messenger of Allah (may peace be upon him) used to observe i'tikaf in the last ten days of Ramadan. Nafi' said: Abdullah (Allah be pleased with him) showed me the place in the mosque where the Messenger of Allah (may peace be upon him) used to observe i'tikaf.