Surah Luqman 31:19

وَٱقْصِدْ فِى مَشْيِكَ وَٱغْضُضْ مِن صَوْتِكَ ۚ إِنَّ أَنكَرَ ٱلْأَصْوَٰتِ لَصَوْتُ ٱلْحَمِيرِ

And be moderate in your pace and lower your voice; indeed, the most disagreeable of sounds is the voice of donkeys."

Translation: Saheeh International

Tafsir

And be modest in your bearing being moderate in it neither dragging slowly nor rushing but peaceful and dignified and lower your voice; indeed the most hideous of voices is the donkey’s voice’ beginning its hideous bray with an exhalation and ending with an inhalation.

Excerpt from Tafsir al-Jalalayn, commentary on 31:19, via Quran.com. The reader has the full commentary and 9 more tafsirs.