Sunan an-Nasa'i 867
سنن النسائي#867SahihIt was narrated that Ibn Buhainah said:"The Iqamah for Subh prayer was said, and the Messenger of Allah (ﷺ) saw a man praying while the Mu'adhdhin saying the Iqamah. He said: 'Are you praying Subh with four Rak'ahs?
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiSahih - Agreed Upon
In-book reference: 10:91