Hadith #2474 (Sunan Abi Dawud)
Book: Fasting (Kitab Al-Siyam)
Narrator(s): Aban bin Salah bin 'Umayr أبان بن صالح بن عمير, Amr bin Muhammad al-Anqazi عمرو بن محمد العنقزي
Grade: Sahih

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 (ﷺ) about it. He said: Observe i’tikaf (i.e. spend a night or a day near the Ka’bah) and fast.

Source: https://parallelquran.com/hadith/sunan-abi-dawud-2474/