Idris al-Awdi said: Sa'id bin Abi Burdah brought out a letter and said: (This is a letter) from Sayyiduna Umar (may Allah be pleased with him) to Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him). In this letter it says: "Proof is upon the one who makes the claim, and the oath is upon the one against whom the claim is made."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 21207