Musnad Abu Ya'la al-Mawsili 154:31

Musa ibn Muhammad ibn Hayyan narrated to us, Ubayd Allah ibn Abd al-Majid narrated to us, Husam ibn Misak narrated to us, Ata ibn Abi Rabah narrated to us, from Ibn Umar: That the Messenger of Allah (peace and blessings of Allah be upon him) would not wake up during any hour of the night except that he would run the siwak over his mouth.