It is narrated from Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) that regarding the saying of Allah Almighty, "Those who humble themselves in their prayers" [Surah al-Mu'minun: 2], khushu' (humility) means that which is in the heart, and that the Muslim keeps his shoulders gentle and does not look around in his prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3518