Sufyan ibn Usayd al-Hadrami reported that he heard the Prophet, may Allah bless him and grant him peace, say, "It is great treachery to tell something to your brother so that he believes you when you are lying to him."  

حَدَّثَهُ أَنَّهُ سَمِعَ النَّبِيَّ ﷺ يَقُولُ كَبُرَتْ خِيَانَةً أَنْ تُحَدِّثَ أَخَاكَ حَدِيثًا هُوَ لَكَ مُصَدِّقٌ وَأَنْتَ لَهُ كَاذِبٌ