Hadithcore
Al-Adab Al-Mufrad, 545

Mu'adh ibn Jabal said, "When you have love for your brother, do not quarrel with him or treat him badly nor ask questions about him, It may be that an enemy of his will appear and say something about him which is not true and will thereby cause a split between you."

حَدَّثَنَا عَبْدُ اللهِ بْنُ صَالِحٍ قَالَ‏:‏ حَدَّثَنِي مُعَاوِيَةُ، أَنَّ أَبَا الزَّاهِرِيَّةِ حَدَّثَهُ، عَنْ جُبَيْرِ بْنِ نُفَيْرٍ، عَنْ مُعَاذِ بْنِ جَبَلٍ أَنَّهُ قَالَ‏:‏ إِذَا أَحْبَبْتَ أَخًا فَلاَ تُمَارِهِ، وَلاَ تُشَارِّهِ، وَلاَ تَسْأَلْ عَنْهُ، فَعَسَى أَنْ تُوَافِيَ لَهُ عَدُوًّا فَيُخْبِرَكَ بِمَا لَيْسَ فِيهِ، فَيُفَرِّقَ بَيْنَكَ وَبَيْنَهُ‏.‏

References2 variants
In-Book Reference
Book 30, Hadith 8
English Reference
Book 30, Hadith 545