"I asked Aishah how the recitation of the Prophet (S) was at night. [Would he recite silently o audibly?] So she said: 'He would do both of those. Sometimes he was silent with his recitation and sometimes it was audible.' So I said: 'All praise is due to Allah, the One who made the matter broad.'"
حَدَّثَنَا قُتَيْبَةُ، حَدَّثَنَا اللَّيْثُ، عَنْ مُعَاوِيَةَ بْنِ صَالِحٍ، عَنْ عَبْدِ اللَّهِ بْنِ أَبِي قَيْسٍ، قَالَ سَأَلْتُ عَائِشَةَ كَيْفَ كَانَ قِرَاءَةُ النَّبِيِّ صلى الله عليه وسلم بِاللَّيْلِ أَكَانَ يُسِرُّ بِالْقِرَاءَةِ أَمْ يَجْهَرُ فَقَالَتْ كُلُّ ذَلِكَ قَدْ كَانَ يَفْعَلُ رُبَّمَا أَسَرَّ بِالْقِرَاءَةِ وَرُبَّمَا جَهَرَ . فَقُلْتُ الْحَمْدُ لِلَّهِ الَّذِي جَعَلَ فِي الأَمْرِ سَعَةً . قَالَ أَبُو عِيسَى هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ غَرِيبٌ .
Tirmizi
The Book on Salat (Prayer) • Hadith 450