Regarding the verse:

‘… Those who are humble in their prayer…’ [Qur’an 23:2]

‘Alī b. Abī Ṭālib [d.40AH/661CE] said:

“It refers to the humility of the heart, that you soften your shoulder for a Muslim (praying next to you), and that you do not look here and there in your prayer.”

[Waki, al-Zuhd #328, ibn al-Mubarak, al-Zuhd with a da’if isnad]

‘Abdullah ibn ‘Umar said:

“When they stood for prayer, they faced forward in their prayer, they lowered their gaze to the place of prostration, and they knew that Allah turned to them and so they did not will left or right.”

[Suyuti, al-Durr al-Manthur, vol. 10, pg. 557.]

(Read on pg 19-20, Ibn Rajab al-Hanbali, The Humility in Prayer. Daar Us-Sunnah Publishers. Birmingham: 2007.)