Ibn '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.
Sahih Muslim Book 14, Hadith 1
← Previous Hadith We were talking about Lailat-ul-Qadr in the presence of the Messenger of Allah (ﷺ) and he said: He who amongst you remembers (the night) when the moon arose and it was like a piece of plate (at the fag end of the month in a state of waning).

Sahih Muslim Book 13, Hadith 285 Next Hadith → Abdullah (Allah be pleased with him) showed me the place in the mosque where the Messenger of Allah (ﷺ) used to observe i'tikaf.

Sahih Muslim Book 14, Hadith 2