Musnad Abu Ya'la al-Mawsili 110:159

Abu Bakr narrated to us, 'Atham bin 'Ali narrated to us, from al-A'mash, from Habib, from Sa'id, from Ibn 'Abbas, who said: The Messenger of Allah (ﷺ) used to pray two rak'ahs, then he would use the siwak.