Musannaf Ibn Abi Shayba 1:1786

Ya'la ibn 'Ubayd narrated to us, from Muhammad ibn Ishaq, from Muhammad ibn Ibrahim, from Abu Salamah ibn 'Abd al-Rahman, from Zayd ibn Khalid al-Juhani, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Were it not that I would burden my nation, I would have commanded them to use the siwak at every prayer." He said: Zayd ibn Khalid used to keep his siwak on his ear, in the position of a pen on the ear of a scribe. He would not stand for prayer without using it, then he would return it to its place.