Tirmidhi Book 1, Hadith 23

حَدَّثَنَا هَنَّادٌ، حَدَّثَنَا عَبْدَةُ بْنُ سُلَيْمَانَ، عَنْ مُحَمَّدِ بْنِ إِسْحَاق، عَنْ مُحَمَّدِ بْنِ إِبْرَاهِيمَ، عَنْ أَبِي سَلَمَةَ، عَنْ زَيْدِ بْنِ خَالِدٍ الْجُهَنِيِّ، قَالَ:‏‏‏‏ سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ:‏‏‏‏ لَوْلَا أَنْ أَشُقَّ عَلَى أُمَّتِي لَأَمَرْتُهُمْ بِالسِّوَاكِ عِنْدَ كُلِّ صَلَاةٍ وَلَأَخَّرْتُ صَلَاةَ الْعِشَاءِ إِلَى ثُلُثِ اللَّيْلِ . قال:‏‏‏‏ فَكَانَ زَيْدُ بْنُ خَالِدٍ يَشْهَدُ الصَّلَوَاتِ فِي الْمَسْجِدِ وَسِوَاكُهُ عَلَى أُذُنِهِ مَوْضِعَ الْقَلَمِ مِنْ أُذُنِ الْكَاتِبِ، ‏‏‏‏‏‏لَا يَقُومُ إِلَى الصَّلَاةِ إِلَّا أُسْتَنَّ ثُمَّ رَدَّهُ إِلَى مَوْضِعِهِ. قَالَ أَبُو عِيسَى:‏‏‏‏ هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ

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.