Muhammad bin Abdur-Rahman bin Thawban (may Allah have mercy on him) said that he heard Ibn Umar (may Allah be pleased with them both) say: "The prayer of the night and the day is two rak'ahs at a time," and by this he meant voluntary prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4576