Ubayd bin Umayr (may Allah have mercy on him) said that Sayyiduna Umar (may Allah be pleased with him) said, regarding a man who takes something in pledge and it is lost while with him: If it is worth less than the debt it secured, (the pledgor) must pay him the rest of his due; and if it is worth more, he is a trustee. This is not well known from Sayyiduna Umar (may Allah be pleased with him), and the reports from Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) on this differ.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرهن / 11228