A man came while the Messenger of Allah ﷺ was performing the dawn prayer. He prayed two units of prayer on his own, then joined the congregation. When the Messenger of Allah ﷺ finished his prayer, he asked, "So-and-so, which of the two was your real prayer, the one you prayed with us or the one you prayed by yourself?"
أخبرنا يحيى بن حبيب بن عربي قال حدثنا حماد قال حدثنا عاصم
جاء رجل ورسول الله ﷺ في صلاة الصبح فركع الركعتين ثم دخل فلما قضى رسول الله ﷺ صلاته قال يا فلان أيهما صلاتك التي صليت معنا أو التي صليت لنفسك
Muslim reported it, hadith: 712. See the preceding hadith transmitted from Hammad bin Zayd, and it is in Al-Kubra, hadith: 940. Asim is Ahwal.