Musannaf Ibn Abi Shayba 3:3346

Khalid ibn Makhlad narrated to us, he said: Thabit ibn Qays narrated to us, he said: I prayed the Fitr [prayer] behind Umar ibn Abd al-Aziz. He said: He performed seven takbirs in the first [rak'ah] before the recitation, and five in the second before the recitation.