(a) Sayyiduna Zayd bin Khalid al-Juhani (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) say: "Had I not considered it a hardship for the Ummah, I would have ordered them to use the siwak for every prayer."
(b) Abu Salamah (may Allah have mercy on him) said: I saw Sayyiduna Zayd (may Allah be pleased with him) sitting in the mosque, and the siwak was on his ear just as a scribe keeps his pen on his ear. Whenever he stood for prayer, he would use the siwak.
It reached me that al-Bukhari used to say: The hadith of Abu Salamah from Zayd bin Khalid is more authentic. Abu Isa al-Tirmidhi said: In my view both of them are sahih (authentic).
The Shaykh (may Allah have mercy on him) said: The last part of this hadith has been reported from Muhammad bin Ishaq bin Yasar (may Allah have mercy on him) through another chain of his.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 156