Imam Ja'far al-Sadiq (may Allah have mercy on him) narrates from his father (Imam Muhammad al-Baqir, may Allah have mercy on him): Sayyiduna Ali (may Allah be pleased with him) said: "The dowry will be that upon which the husband and wife are mutually agreed."
Hadith Referenceسنن الدارقطني / كتاب النكاح / 3607