Sunan an-Nasa'i, Book 21, Hadith 119 (In-Book) ⓘ
It was narrated that 'Aishah said: "Something bad was said in the presence of the Prophet about a person who had died. He said: 'Do not say anything but good about your dead."'
أَخْبَرَنَا إِبْرَاهِيمُ بْنُ يَعْقُوبَ، قَالَ حَدَّثَنِي أَحْمَدُ بْنُ إِسْحَاقَ، قَالَ حَدَّثَنَا وُهَيْبٌ، قَالَ حَدَّثَنَا مَنْصُورُ بْنُ عَبْدِ الرَّحْمَنِ، عَنْ أُمِّهِ، عَنْ عَائِشَةَ، قَالَتْ ذُكِرَ عِنْدَ النَّبِيِّ صلى الله عليه وسلم هَالِكٌ بِسُوءٍ فَقَالَ " لاَ تَذْكُرُوا هَلْكَاكُمْ إِلاَّ بِخَيْرٍ " .
also referenced under: Hadith 1935 (Reference) & Volume 3, Book 21, Hadith 1937 (DarusSalam)