Concerning the saying of Allah Almighty, 'And do not recite your prayer too loudly, nor too quietly' [Surah al-Isra: 110], it is reported from Sayyidah Aishah (may Allah be pleased with her) that what is meant by it is supplication.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3008