Sayyiduna Ibn Abbas (may Allah be pleased with them both) said concerning the saying of Allah Almighty: "And do not be loud in your prayer, nor make it too quiet" [Surah al-Isra: 110]: "That is, one should not pray to show off to people, nor abandon it out of fear of people."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12973