Musnad Ahmad 27:1478

'Abd al-Razzaq narrated to us, Sufyan informed us, from Habib ibn Abi Thabit, from Tawus, from Ibn 'Umar, who said: The Prophet, peace and blessings of Allah be upon him, was asked about the night prayer. He said: "Two by two. Then, if you fear the morning, then one."