Yahya narrates from his father, and he from his grandfather (may Allah be pleased with him), that he came while the Prophet (peace and blessings be upon him) was leading the Fajr prayer, so he prayed with him. When he (peace and blessings be upon him) said the salam, he stood up and prayed the two rak'ahs of Fajr. The Prophet (peace and blessings be upon him) said: "What are these two rak'ahs?" He said: I had not prayed them before Fajr. He (peace and blessings be upon him) remained silent and said nothing.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4553