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
It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that he (peace and blessings be upon him) said: "When one of you gets up at night for prayer, let him begin his prayer with two light rak'ahs."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4670
It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) used to begin his tahajjud with two light rak'ahs.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4671