Allaahu ‘Akbar! Allah is the Most Great. اللهُ أَكْبرُ Al-Bukhari, cf. Al-Asqalani, Fathul-Bari 8/441. See also Al-Albani, Sahih At-Tirmidhi
What to say when you are surprised or startled
Subhaanallaah سُبْحَانَ اللهِ “Glory is to Allah.” Al-Bukhari, cf. Al-Asqalani, Fathul-Bari 1/210, 390, 414 and Muslim 4/1857.