When they both met the Prophet (ﷺ) together, the Prophet (ﷺ) 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, Messenger of Allah. He (the Prophet) said to Umar: I passed by you when you were praying in a loud voice. He replied: Messenger of Allah, I was awakening the drowsy and driving away the Devil.
Al-Hasan added in his version: The Prophet (ﷺ) said: Raise your voice a little, Abu Bakr, and he said to Umar: Lower your voice a little.
In-book : Book 5, Hadith 80
English translation : Book 5, Hadith 1324