Idris al-Awdi (may Allah have mercy on him) said: Sa'id bin Abi Burdah (may Allah have mercy on him) brought out a letter and said: This is the letter of Umar (may Allah be pleased with him) to Abu Musa al-Ash'ari (may Allah be pleased with him). It contained: Give the claimant a time limit; if he brings proof, well and good, otherwise the judgment is given against him. This time limit will remove any excuse he has.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20730