The bondsman of Allah does not truly bow (in ruku') but that Allah adorns him with the light of His radiance, shades him in the shade of His greatness and clothes him in the garment of His purity. Bowing is first and prostration (sujud) is second: there is courtesy in bowing, and in prostration nearness to Allah. Whoever is not good in the courtesy is not fit for nearness; therefore bow with the ruku' of one who is humble to Allah, abased in his heart and fearful under His power, submitting his limbs to Allah like one who is fearful and sorrowful for what he might miss of the benefits of those who bow.
It is related that Rabi' ibn Khuthaym used to stay awake all night until dawn, in a single ruku'. In the morning he would sigh and say, 'Oh! The sincere have gone ahead, and we are cut off!' Make your ruku' properly by keeping your back straight, coming down from your aspirations in standing to serve Him which comes only with His help. Let your heart flee from the whisperings, tricks and deceit of Satan. Allah will elevate His bondsmen according to their humility to Him, and will guide them to the roots of humility, submission and abasement according to how well their innermost being is acquainted with His immensity.