Śatapatha Brāhmaṇa ("of a hundred paths") 5.2.1.9

5.2.1.9

atha niśrayaṇo niśrayati | sa dakṣiṇata udaṅ roheduttarato vā dakṣiṇā dakṣiṇatastvevodaṅ rohettathā hyudagbhavati

He then leans a ladder (against the post). He may ascend either from the south northwards, or from the north southwards; but let him rather ascend from the south northwards (udak), for thus it goes upwards (udak).