Allah (swt) prepared the Prophet Muhammad (saws) for this major mission, namely, bearing the Message of Islam. He (swt) readied him for carrying the Message of Islam by the performance of the night supererogatory prayer and spiritual closeness. This night prayer provided the Prophet (saws) with the necessary energy to shoulder this great responsibility of notifying the people about the Way of Guidance. The (heavy word) in this verse refers to the Divine Revelation. Allah (swt) does not say: ‘We will reveal to you a heavy word), rather He (swt) used the word (cast upon) because it matches the words (heavy word) and denotes hardship and strength. Receiving the Revelation was so difficult for the Messenger Muhammad (saws); sometimes his forehead was dripping with sweat. One of his (swt) Companions narrated that once a time while he was sitting close to the Prophet (saws) and his (saws) knee was on the Companion knee. Thereupon, the Divine Revelation came down upon him (saws) and the Companion felt as if his knee was as heavy as a mountain. So, the Prophet (saws) used to get exhausted while receiving the Divine Revelation, thus when he received it for the first time, he returned quickly to Sayyida Khadija (may Allah be pleased with her), saying to her: “Wrap me, Wrap me”. Since the Divine Revelation was very exhausting and difficult for him (saws), it stops temporarily in order for him to have some rest and regain his energy. Then, after he became relaxed and found pleasure in the Divine Revelation, he longed for more new Revelations. Allah (swt) says:

 أَلَمْ نَشْرَحْ لَكَ صَدْرَكَ(1) وَوَضَعْنَا عَنكَ وِزْرَكَ (2) الَّذِي أَنقَضَ ظَهْرَكَ

(Did We not expand for you, [O Muhammad], your breast? (1) And We removed from you your burden (2) Which had weighed upon your back) (Ash-Sharah: 1-3)

The word (burden) here refers to the burden which the Messenger (saws) used to bear he received the Divine Revelations.