Abu Bakr bin Muhammad bin Amr bin Hazm (may Allah have mercy on him) narrates from his father (may Allah have mercy on him), and he from his grandfather (may Allah be pleased with him), that in the letter which the Prophet (peace and blessings be upon him) wrote concerning blood money (diyah) there was: "Upon the people of gold are 1000 dinars."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الديات / 16179