Isma'il bin Muslim al-Bdy إسماعيل بن مسلم العبدي · The Book on Salat (Prayer)

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."