Musannaf Ibn Abi Shayba 3:3985

Abu Bakr narrated to us, he said: Waki' narrated to us, from Abdullah bin Sa'id, from Salim Abu an-Nadr, from Bishr bin Sa'id, from Zayd bin Thabit, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The best of prayer is a person's prayer in his house, except for the prescribed (obligatory) prayer."