Musnad Ahmad 903:8

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from ‘Asim al-Ahwal, from ‘Abdullah ibn Sarjis, who said: The prayer was called, the morning prayer, and the Messenger of Allah (peace and blessings of Allah be upon him) saw a man praying the two rak‘ahs of the dawn prayer. He said to him: "With which of your two prayers do you count it? With your prayer alone, or your prayer which you prayed with us?"