Shatapatha Brahmana 1.7.1.13

1.7.1.13

tadyathaivādaḥ | somaṃ rājānam pavitreṇa sampāvayantyevamevaitatsampāvayatyudīcīnadaśaṃ vai tatpavitram bhavati yena tatsomaṃ rājānaṃ sampāvayanti tasmādudaṅnidadhyāt

Just as then (i.e. at the Soma-sacrifice) they clarify king Soma with a strainer, in like manner he now clarifies (the milk); and since the strainer wherewith on that occasion they clarify king Soma has its fringe directed towards the north, therefore let him now also put it down with the top northward.