(a) Ibn Abi Musa narrates from his father (may Allah be pleased with him): I came to the Messenger of Allah (peace and blessings be upon him) and found him using a miswak with his hand, and the sound "a' a'" was coming from his (peace and blessings be upon him) blessed mouth.
("A' a'") is the sound that comes out while using the miswak; the miswak was in his (peace and blessings be upon him) mouth, as if he (peace and blessings be upon him) were making a sound.
(b) Abu Musa (may Allah be pleased with him) says: I came to the Prophet (peace and blessings be upon him) while he (peace and blessings be upon him) was using the miswak, and one end of the miswak was on his tongue.
(c) In a hadith of Sahih al-Bukhari the words are "u' u'".
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 142