Qatadah (may Allah have mercy on him) said: It was mentioned to us that Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) bequeathed a fifth of his wealth and said: "From my wealth I am content only with what Allah was content with from the spoils of the Muslims." And Qatadah (may Allah have mercy on him) said: It used to be said: A fifth is reasonable, a quarter is a strain, and a third is what the judges permit.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الوصايا / 12574