Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates this incident from the Prophet (peace and blessings be upon him), but he did not mention that he (peace and blessings be upon him) said to Sayyiduna Abu Bakr (may Allah be pleased with him), "Raise your voice," nor did he mention the words that he said to Sayyiduna Umar (may Allah be pleased with him), "Lower your voice." He (peace and blessings be upon him) said: "O Bilal, I heard that you recite from such-and-such a surah and such-and-such a surah." He said: "It is excellent and pure speech; Allah will join some of it to some other." He (peace and blessings be upon him) said: "You have hit upon what is right."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4702