Sahih Ibn Khuzaymah

It is narrated from Sayyiduna Abu Qatadah (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) passed by Sayyiduna Abu Bakr (may Allah be pleased with him) while he was performing prayer (Tahajjud) in a low voice, and he (peace be upon him) also passed by Sayyiduna Umar (may Allah be pleased with him) who was performing prayer in a loud voice. Then when both of them came together to the Prophet (peace and blessings of Allah be upon him), he (peace be upon him) said to Sayyiduna Abu Bakr (may Allah be pleased with him): "O Abu Bakr, I passed by you while you were praying and reciting in a low voice." He replied, "The One with whom I was conversing has heard me." And (he said), "O Umar, I passed by you while you were reciting in a very loud voice." He replied, "O Messenger of Allah, I hope for reward from this, I wanted to wake up the sleeping one and I hope for reward in this." The Prophet (peace and blessings of Allah be upon him) said to Sayyiduna Abu Bakr (may Allah be pleased with him): "Raise your voice a little." And he said to Sayyiduna Umar (may Allah be pleased with him): "Lower your voice a little." Imam Abu Bakr (may Allah have mercy on him) says: I have mentioned the reason for the revelation of this verse « وَلَا تَجْهَرْ بِصَلَاتِكَ وَلَا تُخَافِتْ بِهَا وَابْتَغِ بَيْنَ ذَٰلِكَ سَبِيلًا » [ سوره الاسراء : 110 ] in the book Al-Imamah.