Musnad Abu Ya'la al-Mawsili 154:2

Al-Hasan ibn Hammad al-Kufi narrated to us, 'Amr ibn Muhammad al-'Anqazi narrated to us, from 'Abdullah ibn Budayl ibn Warqa', from 'Amr ibn Dinar, from Ibn 'Umar: That 'Umar asked the Prophet, peace and blessings of Allah be upon him, about an i'tikaf (seclusion) that was upon him, so he ordered him to perform the i'tikaf and to fast. While he was in i'tikaf, the people shouted "Allahu Akbar." He said, "What is this, O 'Abdullah?" He said, "The captives of Hawazin; the Messenger of Allah, peace and blessings of Allah be upon him, has freed them." 'Umar said, "And that slave girl?" So he sent her with them.