Sayyiduna Sahl bin Sa'd al-Sa'idi (may Allah be pleased with him) said: A woman came to the Prophet (peace and blessings be upon him) and offered herself to him (peace and blessings be upon him) as a gift, and she stood for a very long time. Then a man stood up and said: "O Messenger of Allah (peace and blessings be upon him)! If you have no need of her, marry me (to her)." He (peace and blessings be upon him) asked: "What do you have to give as dower?" The man said: "O Messenger of Allah (peace and blessings be upon him)! I have this lower garment of mine." He (peace and blessings be upon him) said: "If you give the garment, you will have nothing left; look for something else." He said: "I have nothing." He (peace and blessings be upon him) said: "Look, even if it is an iron ring." Despite searching he found nothing, so the Messenger of Allah (peace and blessings be upon him) said: "Do you know any of the Qur'an?" He said: "I know such-and-such surahs." So the Messenger of Allah (peace and blessings be upon him) said: "I have married you to her in return for that Qur'an." In the narration of Abu Hazim it is: "Even if it is only an iron ring."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصداق / 14358