Translation of the verse 5 from Surah Al-Muzzammil : Number of verses 20 - - page 574 - Part 29.
Verily, We shall send down to you a weighty Word (i.e. obligations, legal laws, etc.).
Indeed, We will cast upon you a heavy word.
(73:5) Behold, We shall cast upon you a Weighty Word. *5
*5) That is, "You are being commanded to stand up in the Night Prayer because We are going to send down on you a weighty word, to bear the burden of which you must develop necessary power in yourself, and you can develop this power only by abandoning your ease and comfort of the night and by standing up in the Prayer and passing half the night or thereabout in the worship of your Lord. "The Qur'an has been called a weighty Word also for the reason that acting on its commands, demonstrating its teaching practically, extending its invitation in the face of the whole world, and bringing about a revolution in the entire system of belief and thought, morals and manners, civilization and social life, according to it, is indeed the weightiest task any human being ever has been charged with: It has been called a weighty Word also because bearing the burden of its revelation was a difficult and heavy duty. Hadrat Zaid bin Thabit says: "Once Revelation came down upon the Holy Prophet (upon whom be peace) in a state when he was resting his head upon my knee. I felt such a pressure of the weight on my knee that I thought it would break." Hadrat `A`ishah says: "I have seen the state of the Holy Prophet's receiving Revelation during intense cold, drops of perspiration started falling from his forehead." (Bukhari, Muslim, Malik, Tirmidhi, Nasa'i): In another tradition Hadrat `A`ishah has stated: "Whenever Revelation came down on the Holy Prophet (upon whom be peace) while he was riding on his she-camel, the she-camel would be forced to rest her chest on the ground and could not move until the Revelation was over." (Musnad Ahmad. Hakim, Ibn Jarir).
Source : Al-Muzzammil Verse 5: Indeed, We will cast upon you a heavy word.