كَبُرَتْ خِيَانَةً أَنْ تُحَدِّثَ أَخَاكَ حَدِيثًا هُوَ لَكَ مُصَدِّقٌ وَأَنْتَ بِهِ كَاذِبٌ
It is a grave betrayal to tell your brother something when he believes you, although you are lying to him.
Source: رواه أبو داود
كَبُرَتْ خِيَانَةً strongly condemns betrayal. أَنْ تُحَدِّثَ أَخَاكَ means “that you tell your brother,” and هُوَ لَكَ مُصَدِّقٌ means “while he believes you.” The wrong is not merely a mistaken detail: it is knowingly exploiting another person's trust.
خِيَانَةً is an accusative specification (tamyīz) describing the gravity of the act. وَأَنْتَ بِهِ كَاذِبٌ is a circumstantial nominal clause: the listener trusts, while the speaker knows he is lying. The contrast deepens the meaning.