The basic meaning of khushu’ is the softness of the heart, its being gentle, still, submissive, broken and yearning. The Messenger of Allah (s.a.w.) said, “Of a surety there is a morsel of flesh in the body, if it is sound, the whole body will be sound, but if it is corrupt, the whole body will be corrupt. It is the heart.” (Bukhari and Muslim). When the heart is humble, so too is the hearing, seeing, head and face; indeed all the limbs and their actions are humbled, even speech. This is why the Prophet (s.a.w.) would say in his bowing (ruku’), “My hearing, sight, bones, and marrow are humbled to You.” (Muslim) One of the Salaf saw a man fidgeting in his prayer and remarked, “If the heart of this person was humble, so too would his limbs be.”
The source of khushu’ that takes place in the heart is the gnosis (knowledge) of Allah’s greatness, magnificence and perfection. The more gnosis a person has of Allah, the more khushu’ he has. The hearts vary in their khushu’ in accordance to the gnosis they have of the One they have humbled to, and in accordance to the hearts’ witnessing the Attributes that lead to khushu’. Some hearts are humbled by the strength of their perceiving His closeness to His servants and His seeing their innermost secrets which lead to being shy of Him, Most High, and constantly being aware of Him in every state of motion and stillness. Some hearts are humbled through their perceiving the magnificence of Allah, His greatness, and His grandeur which leads to being in awe of Him and magnifying Him. Some hearts are humbled through perceiving His perfection and beauty which leads to drowning in the love of Him and the desire to meet and see Him. Some hearts are humbled through perceiving the enormity of His seizure, vengeance and punishment which leads to fearing Him.
He, Glorious is He, is the mender of hearts that have broken for His sake. He, Glorious is He, comes close to hearts that are filled with humility in the same way that He comes close to one who is standing in prayer, privately discoursing with Him (hadith reported by Hakim and authenticated by Albani); in the same way that He comes close to one who rubs his face in the dust when prostrating (Muslim); in the same way that He draws near to the throngs of people visiting His House standing in abject humility at ‘Arafah, drawing close and boasting about them to the angels (Muslim and Ibn Hibban); in the same way that He comes close to His servants when they invoke Him, ask of Him, and seek His forgiveness in the early hours of the morning, and He answers their supplications and grants their requests (Bukhari and Muslim).
– Ibn Rajab al-Hanbali in The Humility in Prayer