Sunan an-Nasa'i, Hadith 867 ⓘ
It was narrated that Ibn Buhainah said: "The Iqamah for Subh prayer was said, and the Messenger of Allah saw a man praying while the Mu'adhdhin saying the Iqamah. He said: 'Are you praying Subh with four Rak'ahs?"'
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ سَعْدِ بْنِ إِبْرَاهِيمَ، عَنْ حَفْصِ بْنِ عَاصِمٍ، عَنِ ابْنِ بُحَيْنَةَ، قَالَ أُقِيمَتْ صَلاَةُ الصُّبْحِ فَرَأَى رَسُولُ اللَّهِ صلى الله عليه وسلم رَجُلاً يُصَلِّي وَالْمُؤَذِّنُ يُقِيمُ فَقَالَ " أَتُصَلِّي الصُّبْحَ أَرْبَعًا " .
also referenced under: Book 10, Hadith 91 (In-Book) & Volume 1, Book 10, Hadith 868 (DarusSalam)