Musannaf Ibn Abi Shayba 3:4257

Abu Bakr narrated to us, he said: Waki’ and Ghundar narrated to us, from Shu’bah, from Ya’la ibn ‘Ata, from ‘Ali al-Azdi, from Ibn ‘Umar, from the Prophet (peace and blessings of Allah be upon him), who said: "The prayer of the night and the day is two rak'ahs, two rak'ahs." Except that Ghundar said: "Two by two."