Imam al-Shafi'i (may Allah have mercy on him) said: Sufyan bin Uyaynah (may Allah have mercy on him) narrated a similar hadith to me with the same chain, except that he said: the Messenger of Allah (peace and blessings be upon him) said, and he did not say that it was of iron, and "he tries to widen it but it does not widen" he said only once.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7811