Sunan at-Tirmidhi (Jami-al-Tirmidhi), Chapter: Taharah (Purification)

reported that Zayd ibn Khalid (RA) al?Juhanni said that he heard Allah's Messenger (SAW) say, "Were it not that I might distress my Ummah I would order them to use the siwak before every salah, and I would put off the salah of isha till one?third of the night had passed” He said, "Zayd ibn Khalid had the siwak on his ear as a scribe has a pen over it when he came to the mosque for his salah. He did not offer salah till he had used the siwak, after which put it back in the same place." [Ahmed17045, Abu Dawud 47]