Is it this one?
رَبَّنَا لاَ تُؤَاخِذْنَا إِن نَّسِينَا أَوْ أَخْطَأْنَا رَبَّنَا وَلاَ تَحْمِلْ عَلَيْنَا إِصْرًا كَمَا حَمَلْتَهُ عَلَى الَّذِينَ مِن قَبْلِنَا رَبَّنَا وَلاَ تُحَمِّلْنَا مَا لاَ طَاقَةَ لَنَا بِهِ وَاعْفُ عَنَّا وَاغْفِرْ لَنَا وَارْحَمْنَا أَنتَ مَوْلاَنَا فَانصُرْنَا عَلَى الْقَوْمِ الْكَافِرِينَ
"Rabbana la tuakhidhna in-naseena aw akhta'nâ.
Rabbana wala tahmil 'alayna isran kama hamaltahu 'alal-ladheena min qablina.
Rabbana wala tuhammilna ma la taqata lana bihi w'afu anna waghfir lana wairhamna anta mawlaana fansurna 'alal-qawmil-kafireen."
"Our Lord! Condemn us not if we forget or fall into error; our Lord! Lay not on us a burden Like that which Thou didst lay on those before us; Our Lord! Lay not on us a burden greater than we have strength to bear. Blot out our sins, and grant us forgiveness. Have mercy on us. Thou art our Protector; Help us against those who stand against faith."
(2:286)
