(Mu'amin) 40 : 84
فَلَمَّا رَأَوْا بَأْسَنَا قَالُوا آمَنَّا بِاللَّهِ وَحْدَهُ وَكَفَرْنَا بِمَا كُنَّا بِهِ مُشْرِكِينَ
Thus when they saw Our scourge they said: Indeed we believed with Allah, the Single, and We denied with what We were associating others with Him.
See explanation 10: 98; 20: 134-135, and 21: 14.