Ramayana
यस्तयोः पूर्वजो जातः स कुशैर्मन्त्रसत्कृतैः । निर्मार्जनीयस्तु तदा कुश इत्यस्य नाम तत् ॥ 7-66-7 ॥
He of the two who was born the elder was to be wiped clean with kusha blades hallowed by sacred mantras, and so his name came to be Kusha.
english translation