Maliks Muwatta Book 8, Hadith Number 4.
Section : The Superiority of Prayer in Congregation over Prayer Done Alone.
Yahya related to me from Malik from Abu’n-Nadr, the mawla of Umar ibn Ubaydullah from Yusr ibn Said that Zayd ibn Thabit said, “The most excellent prayer is your prayer in your house, except for the prescribed prayers.”