Abu Dawud
Fasting
Book Number: 007 / Hadith Number: 2468
Narated By Abdullah ibn Umar : Umar (may Allah be pleased with him) took a vow in the pre-Islamic days to spend a night or a day in devotion near the Ka'bah (in the sacred mosque). He asked the Prophet (pbuh) about it. He said: Observe i'tikaf (i.e. spend a night or a day near the Ka'bah) and fast.