Surah Luqman (Luqman), Verse 19

وَاقْصِدْ فِیْ مَشْیِكَ وَاغْضُضْ مِنْ صَوْتِكَ اِنَّ اَنْكَرَ الْاَصْوَاتِ لَصَوْتُ الْحَمِیْرِ 

Waqsid fee mashyika waghdud min sawtik; inna ankaral aswaati lasawtul hameer

“"And be moderate in thy pace, and lower thy voice; for the harshest of sounds without doubt is the braying of the ass."”

Quran 31:19 (Yusuf Ali Translation)