It is only a revelation revealed,
means, he only conveys to the people what he was commanded to convey, in its entirety without additions or deletions.
Imam Ahmad recorded that Abu Umamah said that he heard the Messenger of Allah say,
لَيَدْخُلَنَّ الْجَنَّةَ بِشَفَاعَةِ رَجُلٍ لَيْسَ بِنَبِيَ مِثْلُ الْحَيَّيْنِ أَوْ مِثْلُ أَحَدِ الْحَيَّيْنِ رَبِيعَةَ وَمُضَر
Verily, numbers similar to the two tribes, or one of them, Rabi`ah and Mudar, will enter Paradise on account of the intercession of one man, who is not a Prophet.
A man asked, "O Allah's Messenger! Is not Rabi`ah a subtribe of Mudar."
The Prophet said,
إِنَّمَا أَقُولُ مَا أَقُول
I said what I said.
Imam Ahmad recorded that Abdullah bin `Amr said,
"I used to record everything I heard from the Messenger of Allah so it would be preserved. The Quraysh discouraged me from this, saying, `You record everything you hear from the Messenger of Allah, even though he is human and sometimes speaks when he is angry.'
I stopped recording the Hadiths for a while, but later mentioned what they said to the Messenger of Allah, who said,
اكْتُبْ فَوَ الَّذِي نَفْسِي بِيَدِهِ مَا خَرَجَ مِنِّي إِلاَّ الْحَق
Write! By He in Whose Hand is my soul, every word that comes out of me is the Truth."
Abu Dawud also collected this Hadith