Abu al-Siddiq al-Naji (may Allah have mercy on him) said that Ibn Umar (may Allah be pleased with them both) saw some people lying down after praying two rak'ahs, before the Fajr prayer. He said: Go and ask them what has prompted them to do this. I went and asked them, and they said: We intend (to follow) the Sunnah. Ibn Umar (may Allah be pleased with them both) said: Go and tell them: This is an innovation (bid'ah).
(Note) Imam al-Shafi'i (may Allah have mercy on him) said that what is intended is a separation between the obligatory and the voluntary prayer; this can be done by lying down, by talking, or by moving from that place; lying down is not specifically required.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4895