This 3 line verse from the Quran warns that some people claim to believe when with believers but admit they do not truly believe when alone with non-believers, saying they were just joking with the believers.

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When they meet the faithful they say: "We
believe;" but when alone with the devils (their
fellows), they say: "We are really with you; we were
joking.".
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(Quran Surah 2, Ayah 14)