The duas of rukoo' are narrated in different sahih hadiths in four ways:
(اَللَّهُمَّ) رَبَّنَا لَكَ الْحَمْدُ
(اَللّٰهُمَّ) 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.
(اَللَّهُمَّ) رَبَّنَا وَلَكَ الْحَمْدُ
(اَللّٰهُمَّ) 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.
رَبَّنَا وَلَكَ الْحَمْدُ، حَمْدًا كَثِيرًا طَيِّبًا مُبَارَكًا فِيهِ
رَبَّنَا وَلَكَ الْحَمْدُ، حَمْدًا كَثِيرًا طَيِّبًا مُّبَارَكًا فِيْهِ
Rabbanaa wa lakal-hamd, hamdan katheeran tayyiban mubaarakan feeh
Our Lord, praise is Yours, abundant, holy and blessed praise.
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