Sufyan narrates from Abd al-A'la: I came to Abu Abd al-Rahman al-Sulami (may Allah have mercy on him). He had made a kitabah contract with his slave for four thousand and stipulated that if he became unable (to pay), he would become a slave again, and whatever I have taken would be mine. Then he reduced one thousand of the four thousand for him and said: I heard your friend Sayyiduna Ali (may Allah be pleased with him) say that what is meant by this saying of Allah Almighty, "And give them from the wealth of Allah which He has given you" [Surah al-Nur: 33], is a quarter.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21670