Musannaf Ibn Abi Shayba 1:1789

'Atham (Ghannam) ibn 'Ali narrated to us, from al-A'mash, from Habib, 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, then he would use the siwak.