Fred, you asked for ‘any text from the Koran or a writing binding on Muslims’ and those words are indeed in the Hadith – “regarded by traditional Islamic schools of jurisprudence as important tools for understanding the Quran and in matters of jurisprudence. Hadith were evaluated and gathered into large collections during the 8th and 9th centuries. These works are referred to in matters of Islamic law and history to this day.” (That copied from Wiki). Whilst some societies do not consider these texts binding in Islamic law, others do, and this and other sayings attributed to Mohammed is the justification cited by Muslims who commit this atrocity. The recently ousted religious fanatics who make up the Muslim Brotherhood are intent on overturning the ruling out-lawing in this practice in Egypt.
There is no doubt that this is being justified on religious grounds. Amnesty International says that the practice has acquired a religious dimension, and Sheikh Yussef al-Badri, a cleric who has repeatedly petitioned courts to make female genital mutilation legal again in Egypt says, “Circumcision' is ordered by Allah, Sharia [Islamic religious law] from Allah. Orders from Allah must be realised”
http://www.bbc.co.uk/news/world-middle-east-22975400
As I said, circumcision is not mentioned in the Koran – but it is mentioned in the Hadith – and therefore, for those who champion this appalling practice, Mohammed’s word does overrule the Koran – although they would never admit it.