Śatapatha Brāhmaṇa ("of a hundred paths") 1.7.3.25

1.7.3.25

dvādaśasvādadhita | dvādaśākṣarā vai jagatī jagatyaivaitaddivamupotkrāmati nātra mātrāsti yatraiva svayam manasā manyeta tadādadhīta sa yadvā apyalpakamiva prāñcamuddharati tenaiva divamupotkrāmati

Or he may lay it down at the distance of twelve steps; for of twelve syllables, indeed, consists the gaga: hence he thereby ascends to heaven by means of the gagatî. Here, however, there is no (fixed) measure: let him, therefore, lay it down where in his own mind he may think proper. If he takes it ever so little east (of the Gârhapatya), he ascends to heaven by it.