Musnad Abu Ya'la al-Mawsili 153:194

Ibrahim ibn al-Hajjaj al-Sami narrated to us, Wahib narrated to us, from Qudamah ibn Musa, from Ayyub ibn Husayn al-Tamimi, from Abu Alqamah, the freed slave of Ibn Abbas, from Yasar, the freed slave of Ibn Umar, who said: Ibn Umar saw me while I was praying after the rising of the dawn. He said: "O Yasar, how many [units] did you pray?" I said: "I do not know." He said: "May you not know! Indeed, the Messenger of Allah (ﷺ) came out to us while we were performing this prayer and said: 'Let those of you who are present inform those who are absent: there is no prayer after the dawn except two prostrations.'"