Musannaf Ibn Abi Shayba 3:838

Narrated to us Kathir ibn Hisham, from Ja'far ibn Burqan, who said: Umar ibn Abd al-Aziz wrote to us: "As to what follows: Indeed, the might of the religion and the foundation of Islam are faith in Allah, the establishment of prayer, and the giving of zakat. So perform the prayer at its time, and be constant in it."