A man came while the Prophet ﷺ was praying the dawn prayer. He prayed two units on his own, then joined the Prophet ﷺ in the prayer. When the Prophet ﷺ finished, he said, "So-and-so, which of the two was your prayer, the one you prayed alone or the one you prayed with us?"
حدثنا سليمان بن حرب حدثنا حماد بن زيد عن عاصم
جاء رجل والنبي ﷺ يصلي الصبح فصلى الركعتين ثم دخل مع النبي ﷺ في الصلاة فلما انصرف قال يا فلان أيتهما صلاتك التي صليت وحدك أو التي صليت معنا
Muslim narrated it in "Prayer of the Traveler," chapter, "Disliking the Beginning of a Voluntary Prayer After the Muezzin Has Begun," etc., hadith no. 712, from the hadith of Hammad bin Zayd, with this chain.