Abu Khalid al-Walibi said that when Abu Hurayrah (may Allah be pleased with him) stood in prayer at night, he would sometimes recite aloud and sometimes quietly, and he would say that the Prophet (peace and blessings be upon him) also used to do the same.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4711