Musnad Ahmad 27:1361

'Affan narrated to us, Wuhayb narrated to us, Qudamah ibn Musa narrated to us, Ayyub ibn Husayn al-Tamimi narrated to us, from Abu 'Alqamah, the freed slave of 'Abdullah ibn 'Abbas, from Yasar, the freed slave of 'Abdullah ibn 'Umar, who said: Ibn 'Umar saw me while I was praying after the dawn had risen. He said: "O Yasar, how many [units] have you prayed?" I said: "I do not know." He said: "May you not know! Indeed, the Messenger of Allah (peace and blessings of Allah be upon him) 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 morning [prayer] except two prostrations.'"