Is Allah's saying: "Indeed, We will soon cast upon you a weighty word" specific to the Prophet Muhammad, peace and blessings be upon him, or does it include everyone who performs the prayer mentioned in the Surah? And is Surah Al-Muzzammil addressed only to the Prophet or to believers in general?

The address in the verse (إِنَّا سَنُلْقِي عَلَيْكَ قَوْلًا ثَقِيلًا) "Indeed, We will cast upon you a weighty word" is specific to the Prophet, peace and blessings be upon him, just as night prayer was obligatory for him alone, and its obligation continued for him even after the فرض (imposition) of the five daily prayers, in exaltation of his status.

The exegetes have differed on the meaning of "weighty word." Some say it refers to the obligation of night prayer and the hardship accompanying it, while others say it refers to the Qur'an, with its injunctions and limits, lawful and unlawful, promise and warning, or that which is heavy upon the hypocrites and disbelievers.