Sunan At-Tirmidhi

Abu Qatadah narrated that :The Prophet (S) said to Abu Bakr: ""I passed by you while you were reciting and your voice was low."" He said: ""I let He who, I was consulting hear."" He said: ""Raise your voice."" Then he said to Umar: ""I passed by while you were reciting and your voice was loud."" So he said: ""I repel drowsiness and keep Ash-Shaitan away."" So he said: ""Lower your voice