Musnad Ahmad 27:1958

'Abd al-Samad and 'Affan narrated to us, they said: Hammad ibn Salamah narrated to us, Ayyub informed us, from Nafi', from Ibn 'Umar, that 'Umar, may Allah be pleased with him, asked the Messenger of Allah, peace and blessings of Allah be upon him, at al-Ji'ranah, and said: "I had vowed during the Jahiliyyah to perform i'tikaf in the Sacred Mosque." 'Abd al-Samad said: And with him was a young boy from the captives of Hawazin. He said to him: "Go and perform i'tikaf." So he went and performed i'tikaf. While he was praying, he heard the people saying: "The Messenger of Allah, peace and blessings of Allah be upon him, has freed the captives of Hawazin." So he called the young boy and freed him.