The word khushu’, humility, implies two meanings: One is humility and submissiveness, while the other is calmness and tranquility, which require a tenderness of the heart that is contrary to harshness. Thus, a humble heart implies its worship of Allah as well as tranquility; hence, humbling oneself in prayer implies both humility and tranquility. The word ‘humble’ cited in the verse “Those [the believers] who humble themselves in their prayers” (23:2) means to Ibn Abbas that the believers are in a state of submissiveness; according to al-Hassan and Qatadah, they are in a state of fear while praying before Allah. Also they are, according to Muqatil, in a state of humility. To Ali ibn Abi Talib this means humility in the heart, showing that one’s disposition should should be tender toward fellow Muslims, and that while praying one should not turn to the right or to the left. Mujahid said it means to lower the gaze and to be responsive. When a man gets up and is ready to pray he will be in a state of humbling himself to Allah, his gaze will not wander, and he will not busy himself thinking about worldly matters.
‘Amr Ibn Dinar stated that the word humility does not only mean bowing down or prostrating but it means tranquility and perfection of performance in prayer. Ibn Sirin and others narrated that while praying the Prophet (pbuh) and his Companions used to raise their eyes toward the sky and turn to the right and left until the following verse was revealed. Allah says: “The believers are indeed the successful ones. Those who humble themselves in their prayers” (23:1-2). Afterward they turned their faces toward wherever they prostrated and, from then on, none of them was seen looking at anything except the ground where they prostrated. ‘Ata’ said that being humble in prayer means not making any gesture with any part of your body while praying. When the Prophet (pbuh) saw a man making a gesture with his beard while praying, he said: “Had he had a humble heart, all the parts of his body would have been humble too.”
Source: Kitab Al-Iman, Pgs. 42-43