Reference : Hisn al-Muslim 241

اللهُ أَكْـبَر
Allāhu Akbar!

(Allah is the Most Great)

Reference:
Al-Bukhari, cf. Al-Asqalani, Fathul-Bari 8/441. See also Al-Albani, Sahih 
At-Tirmidhi 2/103, 235, Ahmad 5/218.

Chapter: At times of amazement and that which delights