It is reported about Sayyiduna Ali (may Allah be pleased with him) that he commanded the use of the miswak and stated that the Prophet (peace and blessings be upon him) said: "When a servant uses the miswak and then stands up to pray, an angel stands behind him and listens attentively to his recitation. He draws close to him (or the narrator used some similar word) until the angel places his mouth upon the person's mouth, and whatever part of the Qur'an comes out of that person's mouth goes inside the angel. So keep your mouths pure and clean for the Qur'an."
Al-Bazzar recorded it, and its narrators are thiqat (trustworthy). (Allamah al-Haythami says:) I say: Ibn Majah recorded part of it, but that report is mawquf (attributed to a Companion), while this one is marfu' (attributed to the Prophet).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2564