Sunan Ibn Majah 276
سنن ابن ماجه#276SahihIt was narrated from Abu Sa'eed Al-Khudri that:The Messenger of Allah said: "The key to prayer is purification, its opening is to say Allahu Akbar and its closing is to say As-salamu 'alaikum
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniSahih
Muhammad Fouad Abd al-BaqiSahih
Shuaib Al ArnautSahih Lighairihi
Zubair Ali ZaiHasan
In-book reference: 1:10