Musnad Ahmad 27:673

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Ya'la ibn 'Ata, that he heard 'Ali al-Azdi narrating, that he heard Ibn 'Umar narrating, from the Prophet, may Allah bless him and grant him peace, that he said: "The prayer of the night and the day is two by two." And Shu'bah used to separate them.