Ibn Shihab (may Allah have mercy on him) narrates from Sayyiduna Zayd bin Thabit (may Allah be pleased with him): When the man goes in to the woman and the curtains are let down, the dower (mahr) has become obligatory.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصداق / 14480