Truth of the Message, Surah Al-Kahf:110

In the Name of Allah, The Most Gracious, The Most Merciful.

قُلْ إِنَّمَآ أَنَاْ بَشَرٌ مِّثْلُكُمْ يُوحَى إِلَىَّ أَنَّمَآ إِلَـهُكُمْ إِلَـهٌ وَاحِدٌ فَمَن كَانَ يَرْجُو لِقَآءَ رَبِّهِ فَلْيَعْمَلْ عَمَلاً صَـلِحاً وَلاَ يُشْرِكْ بِعِبَادَةِ رَبِّهِ أَحَدَا
Say: "I am only a man like you. It has been revealed to me that your God is One God. So whoever hopes for the meeting with his Lord, let him work righteousness and associate none as a partner in the worship of his Lord.''

-This is the last ayah of Surah Al-Kahf, which talks about the stories of the people of cave, Dhul Qarnain etc.
-Prophet Muhammed sallallahu alihiwassalam is only a man, and he knew only what Allah SWT made known to him. He was an illiterate man, and Qur'an being revealed to him, is the wisdom of Allah SWT. Had he been among the educated people in the tribe of Quraysh, the eloquent words of The Lord would have been subjected to more doubt (whether it is the word of Muhammed or The Lord Almighty). So this is a proof that Qur'an is truly the words of Allah. And hence, Allah challenges to bring something similar like the Qur'an for those who doubt. And no one will be able to bring something so true and never-ending in the guidance.
- La ilaha illa Allah- There is no deity worthy of worship except Allah. Nothing or no one has the right to be associated to Him.
- Let him work righteousness- if one hopes for the meeting with His Lord, and a good reward and recompense, then he should be righteous only to seek the pleasure of Allah, and no one else.
-Here righteousness means practising dheen in accordance with the prescribed laws of Allah Azza Wajal.
-Associate none as a partner to Allah: Purity of intention should be there in whatever we do, it should be only for the sake of earning the pleasure of Allah (not to show others or for humanitarian cause), and in accordance with the ways of the Messenger Sallallahu alihiwassalam. Imam Ahmad recorded that Mahmud bin Labid said that the Messenger of Allah said: "What I fear the most for you is the small Shirk." They said: "What is the small Shirk, O Messenger of Allah'' He said, "Showing off (Ar-Riya')". Allah will say on the Day of Resurrection, when the people are rewarded or punished for their deeds, "Go to the one for whom you were showing off in the world and see if you will find any reward with him.''

And Allah knows best.