Surah Luqman (31:19)

وَٱقْصِدْ فِى مَشْيِكَ وَٱغْضُضْ مِن صَوْتِكَ إِنَّ أَنكَرَ ٱلْأَصْوَٰتِ لَصَوْتُ ٱلْحَمِيرِ

And aim straight in your walking, and lower (some) of your voice; indeed, the most disowned of voices is surely the voice of donkeys.