Sa'id bin al-Musayyib (may Allah have mercy on him) said that the Messenger of Allah (peace and blessings be upon him) said: "There is no prayer after the call (adhan) except two rak'ahs," meaning (the adhan of) Fajr.
It has also been narrated with a connected chain mentioning Abu Hurayrah, but its being connected is not sound.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4444