Ayyub said: I saw with Thumamah bin Abdullah bin Anas (may Allah be pleased with him) a document that Abu Bakr al-Siddiq (may Allah be pleased with him) had written for Anas (may Allah be pleased with him) when he sent him to collect zakah from Bahrain. It bore the seal of the ring of the Prophet (peace and blessings be upon him), on which was written 'Muhammadun Rasulullah' (Muhammad is the Messenger of Allah).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7249