Sayyiduna Ibn Umar (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) said: "The prayers of the day and night are in sets of two rak‘ahs." Some narrators said: "Two by two rak‘ahs."
Hadith Referenceسنن دارمي / من كتاب الصللاة / 1497