Silsilat al-Ahadith al-Sahihah

Narrated from Sayyiduna Anas Sa'd (may Allah be pleased with him) that when the Prophet (peace and blessings be upon him) was residing, he would observe i'tikaf during the last ten days of Ramadan, and if he was on a journey during those days, then in the following year he would observe i'tikaf for twenty days.