It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that two men came to the Prophet (peace and blessings be upon him), and both of them had the same need. One of them spoke, and the Messenger of Allah (peace and blessings be upon him) noticed a bad smell from him. He (peace and blessings be upon him) said to him: "Do you not use the siwak?" He said: Of course, but I have not eaten anything for three days. He (peace and blessings be upon him) ordered a man, who helped him and fulfilled his need.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 168