Sahih Muslim Book 27, Hadith 40
This hadith is transmitted on the authority of Ibn Umar with a slight variation of words.
Messenger of Allah, I had taken a vow during the days of Ignorance (Jahiliyya) that I would observe I'tikaf for a night in the Sacred Mosque. He (the Holy Prophet) said: Fulfil your vow.
Messenger of Allah, I had taken a vow during the days of Ignorance that I would observe I'tikaf for one day in the Sacred Mosque. So what is your opinion? He said: Go and observe I'tikaf for a day. And Allah's Messenger (ﷺ) gave him a slave girl out of the one-fifth (of the spoils of war meant for the Holy Prophet). And when Allah's Messenger (inay peace be upon him) set the war prisoners free. 'Umar b. Khattab heard their voice as they were saying: Allah's Messenger (ﷺ) has set as free. He (Hadrat 'Umar) said: What is this? They said: Allah's Messenger (ﷺ) has set free the prisoners of war (which had fallen to the lot of people). Thereupon he (Hadrat 'Umar) said: Abdullah, go to that slave-girl and set her free.