Yasar, the freed slave of Ibn Umar (may Allah be pleased with them both), narrates from Ibn Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "After Fajr there is no prayer except two rak'ahs."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4439