From Sahih Muslim (Hadith 2781):
Abdullah b. Umar (Allah be pleased with both of them) reported that the Messenger of Allah (ﷺ) 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 (ﷺ) used to observe i'tikaf
Reference: Muslim 2781