Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said concerning this verse: "And do not recite your prayer aloud, nor in a hushed voice" [Surah al-Isra': 110]: "When he (peace and blessings be upon him) supplicated, he would raise his voice."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11660