Musnad Ahmad 27:473

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Ayyub, from Nafi', from Ibn 'Umar, who said: When the Prophet, peace and blessings of Allah be upon him, returned from Hunayn, 'Umar asked him about a vow he had made during the Pre-Islamic period (Jahiliyyah) to seclude himself (i'tikaf) for a day. He commanded him to fulfill it. Ibn 'Umar set out before him. He said: And he sent with me a slave girl whom he had acquired on the day of Hunayn. He said: I placed her in one of the houses of the Bedouins when I stopped. Then, suddenly, I saw the captives of Hunayn coming out, running, saying: "The Messenger of Allah, peace and blessings of Allah be upon him, has freed us." He said: So 'Umar said to 'Abdullah, "Go and release her." He said: So I went and released her.