The prayer for Fajr was called, and the Messenger of Allah ﷺ saw a man praying while the muezzin was making the iqamah. He said, "Are you praying Fajr as four?"
أخبرنا قتيبة قال حدثنا أبو عوانة عن سعد بن إبراهيم عن حفص بن عاصم
أقيمت صلاة الصبح فرأى رسول الله ﷺ رجلا يصلي والمؤذن يقيم فقال أتصلي الصبح أربعا
Muslim narrated it in Salat al-Musafirin, in the chapter "The Dislike of Beginning a Voluntary Prayer After the Muezzin Has Begun the Call...," hadith 66/711, from Qutayba, and Bukhari narrated it in the Book of Adhan, in the chapter "When the Prayer Has Been Called, No Prayer Should Be Performed Except the Prescribed One," hadith 663, from the hadith of Saad bin Ibrahim, with the same wording. It is also found in Al-Kubra, hadith 939.