Mishkat al-Masabih

Kathir b. ‘Abdallah on his father’s authority quoted his grandfather as saying that at the two festivals the Prophet (ﷺ) said “God is most great” seven times in the first rak'a before reciting from the Qur’an, and five times in the last one before reciting. Tirmidhi, Ibn Majah and Darimi transmitted it.