Śatapatha Brāhmaṇa ("of a hundred paths") 10.2.3.15

10.2.3.15

tadāhuḥ yattrayodaśa puruṣā atiyanti kathamete sampado na cyavanta iti yā vā etasya saptamasya puruṣasya sampatsaivaiteṣāṃ sarveṣāṃ sampat

As to this they say, 'When thirteen man's lengths are over, how is it that these do not deviate from the right proportions (of the altar)?' Well, what right proportions there were in the case of that seventh man's length, these same proportions (also apply) to all these (redundant man's lengths).