(78) باب الاِعْتِكَافِ
Narrated Ubayy ibn Ka'b:
The Prophet (ﷺ) used to observe i'tikaf during the last ten days of Ramadan. One year he did not observe i'tikaf. When the next year came, he observed i'tikaf for twenty nights (i.e. days).
Reference: Sunan Abi Dawud 2463
In-book reference: Book 14, Hadith 151
English translation: Book 13, Hadith 2457