Hadith 3664
Sayyiduna Anas bin Malik (may Allah be pleased with him) narrates: When the Prophet Muhammad (peace and blessings be upon him) was residing (not traveling), he used to observe i'tikaf during the last ten days of Ramadan, and when he was traveling during the last ten days of Ramadan, then in the following year he would observe i'tikaf for twenty (20) days.