The duas of rukoo' are narrated in different sahih hadiths in four ways:

(اَللَّهُمَّ) رَبَّنَا لَكَ الْحَمْدُ

(1) Rabbanaa lakal-hamd (2) Allaahumma Rabbanaa lakal-hamd

[1] Our Lord, to You be the praise. [2] O Allah, our Lord, to You be the praise.

Reference: Bukhari: 796