Kamasutra 3.5.13

3.5.13

tato mātari pitari ca prakāśayet/

When the girl is gained over, and acts openly with the man as his wife, he should cause fire to be brought from the house of a Brahman, and having spread the Kusha grass upon the ground, and offered an oblation to the fire, he should marry her according to the precepts of the religious law. After this he should inform his parents of the fact, because it is the opinion of ancient authors that a marriage solemnly contracted in the presence of fire cannot afterwards be set aside.