Hadith 4:808
Ghundar narrated to us, from Shu'bah, who said: I asked al-Hakam and Hammad about i'tikaf (seclusion in the mosque), and they both said: "One does not perform i'tikaf except in a mosque in which they gather (for congregational prayer)."
حدثنا غندر عن شعبة قال سألت الحكم وحمادا عن الأعتكاف فقالا لا يعتكف إلا في مسجد يجمعون فيه