Yazid bin Abdullah bin Qusayt (may Allah have mercy on him) says: A man was given the good news of a baby girl, and another man said: Give her to me (in marriage as a gift). This was mentioned to Sa'id bin al-Musayyib (may Allah have mercy on him), and he said: "A woman given as a gift (without a dowry) has not been lawful for anyone after the Prophet (peace and blessings be upon him). If he gives her even a whip as a dowry, or more, it is valid." And in another place he said: "If he gives her even a whip as a dowry, she becomes lawful for him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصداق / 14393