Sunan at-Tirmidhi (Jami-al-Tirmidhi), Chapter: Salah (Prayers)

that the Prophet (SAW) said to Sayyidina Abu Bakr (RA), "I passed by you and you were reciting (the Quran) and you had lowered your voice." He said, "I let Him hear Whom I supplicated." But, he said, "Raise (your voice) a little." And, he said to Sayyidina Umar (RA) "I passed by you while you were reciting (the Quran), and you had raised your voice." He said, "I was awakening the sleeping ones and chasing away the devil." But he said, "Lower (your voice) a little."