Surah Al-Baqarah (The Cow), Verse 14

وَاِذَا لَقُوا الَّذِیْنَ اٰمَنُوْا قَالُوْۤا اٰمَنَّا وَاِذَا خَلَوْا اِلٰی شَیٰطِیْنِهِمْ قَالُوْۤا اِنَّا مَعَكُمْ اِنَّمَا نَحْنُ مُسْتَهْزِءُوْنَ ۟

Wa izaa laqul lazeena aamanoo qaalooo aamannaa wa izaa khalaw ilaa shayaateenihim qaalooo innaa ma'akum innamaa nahnu mustahzi'oon

“When they meet those who believe, they say: "We believe;" but when they are alone with their evil ones, they say: "We are really with you: We (were) only jesting."”

Quran 2:14 (Yusuf Ali Translation)