It is narrated from Sayyiduna Hujr bin Qays (may Allah be pleased with him), who had lived in the Jahiliyyah (pre-Islamic era): Sayyiduna Ali (may Allah be pleased with him) asked the Messenger of Allah (peace and blessings be upon him) for the hand of Sayyidah Fatimah (may Allah be pleased with her), and he (peace and blessings be upon him) said: "She is yours, on condition that you treat her well."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3557