Musannaf Ibn Abi Shayba 3:1687

Hussain bin Ali narrated to us, from Za'idah, from Imran bin Muslim, from Suwayd bin Ghaflah, who said: "Whoever enters the mosque while he is in a state of purity remains in a state of I'tikaf (seclusion/devotion) within it as long as he is in it, until he exits from it or breaks his wudu."