Abu Abd al-Rahman al-Sulami narrates from Sayyiduna Ali (may Allah be pleased with him) regarding this saying, "And give them from the wealth of Allah which He has given you" [Surah al-Nur: 33], that what is meant is a quarter.
In the narration of Abu Awanah: a quarter of the kitabah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21669