It is narrated from Sayyiduna Ali (may Allah be pleased with him) that we were ordered to use the siwak, and he said: "When a servant stands to pray, an angel comes to him and stands behind him; he listens to the Qur'an and draws near. He keeps listening and drawing near until he places his mouth upon his mouth, and then every verse he recites goes into the angel's belly."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 162