Musnad Ahmad 939:11

'Ali ibn Thabit narrated to us, from Muhammad ibn Ishaq, from Muhammad ibn Ibrahim ibn al-Harith al-Taymi, from Abu Salamah ibn 'Abd al-Rahman ibn 'Awf, from Zayd ibn Khalid al-Juhani, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Were it not that I would burden my nation, I would have commanded them to use the siwak (tooth stick) at every prayer." He said: Zayd used to go to the mosque with his siwak on his ear in the position of a scribe's pen. No prayer was established but that he used the siwak before he prayed.