Ramayana
यश्चावरो भवेत्ताभ्यां लवेन स समाहितः । निर्मार्जनीयो वृद्धाभिर्लव इत्येव नामतः ॥ 7-66-8 ॥
And he of the two who was the younger, attended carefully, was to be wiped clean by the old women with a lava (a tuft of grass), and so by name he became Lava.
english translation