Śatapatha Brāhmaṇa ("of a hundred paths") 3.2.2.5

3.2.2.5

taddhaike | nakṣatraṃ dṛṣṭvā vācaṃ visarjayantyatrānuṣṭyāstamito bhavatīti vadantastadu tathā na kuryātkva te syuryanmeghaḥ syāttasmādyatraivānuṣṭyāstamitam manyeta tadeva vācaṃ visarjayet

Some, however, make him break his silence on seeing the (first) start, arguing that then the sun has actually set. But let him not do so, for what would become of them if it were cloudy? Let him therefore break silence as soon as he thinks the sun has set.