Musnad Ahmad 25:551

Hassan narrated to us, Zuhair narrated to us, from Qabus, that his father narrated to him, from Ibn Abbas, who said: Two men came to the Prophet of Allah, may Allah bless him and grant him peace, both having the same need. One of them spoke, and the Prophet of Allah, may Allah bless him and grant him peace, perceived an unpleasant odor from his mouth. He said to him: "Will you not use the siwak (tooth stick)?" He replied: "I do indeed do so, but I have not tasted food for three days." So he (the Prophet) ordered a man to take care of him, and he fulfilled his need for him.