Surah Al-Kawthar is the shortest Surah in the Quran.  It contains three verses and its title is taken from the first verse.

This Surah is mentioned in an authentic Hadith, referring to how those that invent new things into the religion, will not be able to drink from the river of Al-Kawthar on the Day of Judgement.

Hadith:

Narrated by Anas ibn Malik (RA): One day when the Prophet (SAW) was still among us, he took a nap, then he raised his head, smiling. We said to him (SAW): ‘Why are you smiling, O Messenger of Allah (SAW)?’

He (SAW) replied: ‘Just now this Surah was revealed to me: In the Name of Allah, the Most Gracious, the Most Merciful. Verily, We have granted you (O Muhammad SAW) Al-Kawthar. Therefore turn in prayer to your Lord and sacrifice (to Him only). For he who hates you, he will be cut off.’

Then he (SAW) said: ‘Do you know what Al-Kawthar is?’ We said: ‘Allah and His Messenger know best.’ He said: ‘It is a river that my Lord has promised me in Paradise. Its vessels are more than the number of the stars. My Ummah will come to me, then a man among them will be pulled away and I will say: “O Lord, he is one of my Ummah” and He will say to me: ‘You do not know what he did (innovated/invented) after you were gone.’

Reference: Sunan-An-Nasa’i Hadith Reference: 904