Wikipedia
Ruku
Rukūʿ refers to the bowing down following the recitation of the Qur'an in the standing position while praying according to Islamic ritual ( salat). There is a consensus on the obligatory nature of the rukūʿ. The position of rukūʿ is established by bending over, putting one's hands on one's knees, and remaining in that position until the individual attains "calmness".
In Al-Ghazali's book Inner Dimensions of Islamic Worship, he wrote about the rukūʿ by saying:
Bowing (rukūʿ) and prostration ( sujud) are accompanied by a renewed affirmation of the supreme greatness of Allah. In bowing you renew your submissiveness and humility, striving to refine your inner feeling through a fresh awareness of your own impotence and insignificance before the might and grandeur of your Lord. To confirm this, you seek the aid of your tongue, glorifying your Lord and testifying repeatedly to His supreme majesty, both inwardly and outwardly. Then you rise from bowing, hopeful that He will be merciful towards you. To emphasise this hope within you, you say samiʾa -llāhu liman ḥamidahu (سمع الله لمن حمده), meaning 'God hears those who give thanks to Him'. Acknowledging the need to express gratitude, you immediately add, rabbana lak al-ḥamd (ربّنا لك الحمد) - 'Grateful praise to You, our Lord!' To show the abundance of this gratitude, you may also say malʾ as-samāwāt wa-malʾ al-ʾarḍ (ملء السماوات وملء الأرض) - 'as much as the heavens and earth contain'.