Musnad Ahmad 32:77

Ibn Abi ‘Adi narrated to us, from Humayd, from Anas, who said: "When the Prophet (peace and blessings of Allah be upon him) was resident, he would perform I’tikaf during the last ten days of Ramadan, and when he traveled, he would perform I’tikaf for twenty [days] the following year." ‘Abdullah ibn Ahmad said: My father said: "I did not hear this hadith except from Ibn Abi ‘Adi, from Humayd, from Anas."