Sayyiduna Ibn Abbas (may Allah be pleased with them both) said that when he (peace and blessings be upon him) prayed the two rak'ahs of Fajr, he would lie down.
Others narrated it from Shu'bah, from Musa, from Sa'id, from the Prophet (peace and blessings be upon him) with a broken chain (munqati').
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4886