Musannaf Ibn Abi Shayba 3:4083

Abu al-Ahwas narrated to us, from Tariq, from Asim ibn Amr, that a group of the people of Iraq came to Umar and asked him about a man’s prayer in his house. Umar said, "No one has asked me about it since I asked the Messenger of Allah, peace and blessings of Allah be upon him, about it. He said: 'A man’s prayer in his house is light, so illuminate your houses.'"