Abu Dawud Book 2, Hadith Number 1324.

Chapter : Recitation of the quran in loud voice during the prayer at night.

Narated By Abu Qatadah : The Prophet (PBUH) went out at night and found Abu Bakr praying in a low voice, and he passed Umar ibn al-Khattab who was raising his voice while praying.

When they both met the Prophet (PBUH) together, the Prophet (PBUH) said: I passed by you, Abu Bakr, when you were praying in a low voice. He replied: I made Him hear with Whom I was holding intimate converse, Apostle of Allah. He (the Prophet) said to Umar: I passed by you when you were praying in a loud voice. He replied: Apostle of Allah, I was awakening the drowsy and driving away the Devil.

Al-Hasan added in his version: The Prophet (PBUH) said: Raise your voice a little, Abu Bakr, and he said to Umar: Lower your voice a little.