Surah Al-Isra 

verses 85-87:

They ask you about the Spirit.
Say: "The Spirit is of my Lord's command, and of knowledge you have been given but little."
And if We willed We could certainly take away that which We have revealed to you;
then you should find none to plead with Us on your behalf.
(All that you have received) is nothing but grace from your Lord.
Indeed his favor to you is great. 

verses 94-95:

And what prevented the people from believing when guidance came to them except that they said, "Has Allah sent a human messenger?"
Say, "Had angels settled on the Earth and moved about in peace, We would certainly have sent an angel as a Messenger to them."