(اَللَّهُمَّ) رَبَّنَا وَلَكَ الْحَمْدُ

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

[1] Our Lord and to You be the praise. [2] O Allah, our Lord and to You be the praise. After rukoo' one should stand straight for a while. It is obligatory upon all. If anyone goes to prostration before stand straight (for a while) then it will invalidate his salaah. In the state of rukoo' one should say one of these four duas. It is recommended to say these supplications alternatively.

Reference: Bukhari: 795