Sayyidah Aishah (may Allah be pleased with her) says that this verse, 'And do not recite your prayer too loudly, nor too quietly' [Surah al-Isra: 110], was revealed concerning supplication.
(b) Likewise, Mujahid said that it was revealed concerning asking in supplication.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3009