Bismillah Hirrahman Nirrahim.
Alhamdulillah.

Abu Huraira radi Alllahu anhu se rivayat hai ki RASOOLALLAH sallAlllahu alaihi wa sallam ne farmaya ki us shaksh ki Namaz nahi jiska Wudhu nahi aur us shaksh ka Wudhu nahi jisney iskey shuru mein BISMILLAH na parhi ho.

English:
Narrated Abu Hurairah 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, Vol 1, 101]