Musannaf Ibn Abi Shayba 23:76

Umar ibn Ayyub narrated to us, from Ja’far ibn Burqan, who said: Umar ibn Abd al-Aziz wrote to us: "As for what follows: Indeed, the bonds of the religion and the foundation of Islam are belief in Allah, the establishment of prayer, and the giving of zakat. So, perform the prayer at its proper time."