Sahih al-Bukhari 1082
صحيح البخاري#1082SahihNarrated `Abdullah bin `Umar:I offered the prayer with the Prophet, Abu Bakr and `Umar at Mina and it was of two rak`at. `Uthman in the early days of his caliphate did the same, but later on he started praying the full prayer
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 18:3