عَنْ نَوَّاسِ بْنِ سَمْعَانَ رَضِيَ اللَّهُ عَنْهُ قَالَ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ ((كَبُرَتْ خِيَانَةً تُحَدِّثُ أَخَاكَ حَدِيثًا هُوَ لَكَ مُصَدِّقٌ وَأَنْتَ بِهِ كَاذِبٌ))
It is narrated from Sayyiduna Nawwas bin Sam'an, may Allah be pleased with him, that the Messenger of Allah, peace and blessings be upon him, said: "It is a great betrayal that you speak to your brother about something and he believes you, while you are lying to him."