Sayyiduna Ibn Umar (may Allah be pleased with them both) narrates from the Noble Prophet (peace and blessings be upon him) that he (peace be upon him) said: "The (voluntary) prayers of the night and day are in sets of two rak‘ahs."
Hadith Referenceصحيح ابن خزيمه / جماع أبواب التطوع غير ما تقدم ذكرنا لها / 1210