وَرَبَّكَ فَكَبِّرْ
Meaning: Magnify your Lord (74:3).
تَحْرِيمُهَا التَّكْبِيرُ وَتَحْلِيلُهَا التَّسْلِيمُ
Meaning: Entry into prayer’s restrictions is by takbir; leaving them is by taslim.
Together these establish the opening takbir’s central place in starting prayer.