Shatapatha Brahmana 9.1.1.28

9.1.1.28

tadvai sahasrayojana iti | etaddha paramaṃ dūraṃ yatsahasrayojanaṃ tadyadeva paramaṃ dūraṃ tadevaiṣāmetaddhanūṃṣyavatanoti

Here now he says 'at a thousand leagues,' for a thousand leagues is the farthest distance; and he thus unstrings their bows at what is the farthest distance.