Musnad Abu Ya'la al-Mawsili 111:69

'Amr ibn Muhammad al-Naqid narrated to us, 'Atham ibn 'Ali narrated to us, al-A'mash informed us, from Habib ibn Abi Thabit, from Sa'id ibn Jubayr, from Ibn 'Abbas, who said: The Messenger of Allah (may Allah bless him and grant him peace) used to pray two rak'ahs at night, then he would turn away and use the siwak.