Sunan Abi Dawud 101
سنن أبي داود#101SahihNarrated AbuHurayrah: The Messenger of Allah (ﷺ) 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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniSahih
Muhammad Muhyi Al-Din Abdul HamidSahih
Zubair Ali ZaiHasan
In-book reference: 1:101