﴿ وَإِذَا لَقُوا۟ ٱلَّذِينَ ءَامَنُوا۟ قَالُوٓا۟ ءَامَنَّا وَإِذَا خَلَوْا۟ إِلَىٰ شَيَٰطِينِهِمْ قَالُوٓا۟ إِنَّا مَعَكُمْ إِنَّمَا نَحْنُ مُسْتَهْزِءُونَ ﴾

“And when they meet those who have attained to faith, they assert, "We believe [as you believe]"; but when they find themselves alone with their- evil impulses, they say, "Verily, we are with you; we were only mocking!"”

Please check ayah 2:15 for complete tafsir.