Sahih Bukhari Volume 004, Book 053, Hadith Number 372.

Narated By Nafi : 'Umar bin Al-Khattab said, "O Allah's Apostle! I vowed to observe Itikaf for one day during the Pre-lslamic period." The Prophet ordered him to fulfill his vow. 'Umar gained two lady captives from the war prisoners of Hunain and he left them in some of the houses at Mecca. When Allah's Apostle freed the captives of Hunain without ransom, they came out walking in the streets. 'Umar said (to his son), "O Abdullah! See what is the matter." 'Abdullah replied, "Allah's Apostle has freed the captives without ransom." He said (to him), "Go and set free those two slave girls." (Nafi added:) Allah's Apostle did not perform the 'Umra from Al-Jarana, and if he had performed the 'Umra, it would not have been hidden from 'Abdullah.