Abdullah bin Rabah narrates from Sayyiduna Abu Qatadah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) passed by Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) while he was reciting in a low voice, and passed by Sayyiduna Umar (may Allah be pleased with him) while he was reciting in a loud voice. When both of them (may Allah be pleased with them both) came to the Prophet (peace and blessings be upon him), he (peace and blessings be upon him) said to Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him): "O Abu Bakr! I passed by you while you were praying with your voice lowered." Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) said: I was making the One with whom I was conversing in secret hear me. He (peace and blessings be upon him) said to Sayyiduna Umar (may Allah be pleased with him): "O Umar! I passed by you while you had raised your voice." He said: O Messenger of Allah! I intended to wake those who were asleep. So he (peace and blessings be upon him) said to Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him): "Raise your voice a little," and said to Sayyiduna Umar (may Allah be pleased with him): "Lower your voice a little."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4700