Musannaf Ibn Abi Shayba 3:4076

Waki’ narrated to us, from ‘Abdullah bin Sa’id, from Salim Abu al-Nadr, from Bishr bin Sa’id, from Zayd bin Thabit, who said: The Prophet (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."