Hadith: The prayer of a person who does not perform ablution is not valid

Narrated AbuHurairah Radi Alllahu Anhu The Messenger of ALLAH peace be upon him said: The prayer of a person who does not perform ablution is not valid, and the ablution of a person who does not mention the name of 

 ALLAH (in the beginning) is not valid. 
Sunan Abu Dawud, Book 1, Number 101 -Sahih