Abu Salih al-Samman (may Allah have mercy on him) said: I heard Sayyiduna Abu Hurayrah (may Allah be pleased with him) telling Marwan bin al-Hakam, who was then governor of Madinah, that the Messenger of Allah (peace and blessings be upon him) used to separate his two rak'ahs of Fajr from the Subh prayer by lying down on his right side.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4888