Taha 20:2

مَا أَنزَلْنَا عَلَيْكَ الْقُرْ‌آنَ لِتَشْقَىٰ (We did not reveal the Qur'an to you to [ make you ] face hardship - 20:2) The word لِتَشْقَىٰ is derived from شقاء which means pain and distress. In the early days of Islam when the Qur'an was first revealed, the Holy Prophet ﷺ and his Companions ؓ used to spend whole nights in prayers and in reciting the Qur'an, as a result of which his feet used to get swollen due to standing for long hours in the prayers. Whereas during day time he would worry himself on how to bring light to the infidels in order that they should accept the teachings of the Qur’ an. This verse seeks to lessen the burden of long hours of prayers by informing the Holy Prophet ﷺ that the Qur’ an was not revealed to inflict toil and hardship on him and that it was not expected of him to stay awake the whole night reciting the Qur'an. Thereafter, the Holy Prophet ﷺ adopted a routine by which he rested in the early hours of the night and would get up later to offer the salah of tahajjud.

This verse also suggests that the duty of the Holy Prophet ﷺ is merely to convey the Message of Allah Ta` ala to the unbelievers, and thereafter he need not concern himself as to who accepted the Message and who did not.