Sahih Muslim
The Book of Oaths (Kitab Al-Aiman)
Book Number: 015 / Hadith Number: 4072
Ibn 'Umar reported that Umar (b. Khattab) said: 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.