Sunan Abi Dawud 61
سنن أبي داود#61SahihNarrated Ali ibn AbuTalib: The key to prayer is purification; its beginning is takbir and its end is taslim
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniHasan Sahih
Muhammad Muhyi Al-Din Abdul HamidHasan Sahih
Zubair Ali ZaiHasan
In-book reference: 1:61