Shatapatha Brahmana 5.2.5.17

5.2.5.17

ṛṣabho vaiśvānarasya dakṣiṇā | saṃvatsaro vai vaiśvānaraḥ saṃvatsaraḥ prajāpatirṛṣabho vai paśūnām prajāpatistasmādṛṣabho vaiśvānarasya dakṣiṇā kṛṣṇaṃ vāso vāruṇasya taddhi vāruṇaṃ yatkṛṣṇaṃ yadi kṛṣṇaṃ na vindedapi yadeva kiṃ ca vāsaḥ syādgranthibhirhi vāso vāruṇaṃ varuṇyo hi granthiḥ

A bull is the fee for the Vaisvânara (oblation); for Vaisvânara is the year, and Pragâpati is the year; and the bull is the Pragâpati (lord of creatures or generation) among cows: therefore a bull is the fee for the Vaisvânara. A black cloth for the Vâruna (oblation), for what is black belongs to Varuna. If he cannot obtain a black one, any kind of cloth will do: it is by its knots that the cloth belongs to Vanilla, for the knot is sacred to Varuna.