"Our Lord, and grant us what You promised us through Your messengers," meaning: through the tongues of Your messengers. God promised the believers, through the tongues of His messengers, that He would admit them into Paradise if they obeyed Him.
﴿رَبَّنَا وَآتِنَا مَا وَعَدْتَنَا عَلَى رسلك﴾ أَي: على أَلْسِنَة رسلك؛ وعد الله الْمُؤمنِينَ على أَلْسِنَة رسله أَن يدخلهم الْجنَّة إِذا أطاعوه.