Sa'id bin al-Musayyib (may Allah have mercy on him) said: Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) gave a ruling concerning a woman whom a man marries: When the curtains are let down, the dower (mahr) becomes obligatory.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصداق / 14479