It is narrated from Sayyiduna Abdullah bin Zubair (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "There is no obligatory prayer except that before it there are (at least) two units (rak‘ahs) of voluntary prayer."
Hadith Referenceسلسله احاديث صحيحه / الاذان و الصلاة / 651