Musannaf Ibn Abi Shayba 9:791

Ibn Fudayl narrated to us, from Hajjaj, from Makhul, who said: A group of the companions of the Prophet, may God bless him and grant him peace, gathered. Umar and Mu'adh said: "Indeed, when the door is closed and the curtain is lowered, the dowry (mahr) has become obligatory."