Jami` at-Tirmidhi 23

Zaid bin Khalid Al-Juhani said :'I heard Allah's Messenger saying: 'If it were not that it would be difficult on my nation then, I would have ordered them to use the Siwak for each prayer and to delay the Isha prayer until the third of the night.'"He [Abu Salamah, one of the narrators] said: Zaid bin Khalid wouId attend the prayer in the Masjid and his Siwak would be on his ear in the location of the pen on the ear of a writer. He would not get up to pray without cleaning his teeth, then returning it to its location