Sa'd bin Hisham (may Allah have mercy on him) says that Sayyidah Aishah (may Allah be pleased with her) said: When the Messenger of Allah (peace and blessings be upon him) stood at night for prayer, he would begin his prayer with two light rak'ahs.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4669