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