Śatapatha Brāhmaṇa ("of a hundred paths") 3.3.3.18

3.3.3.18

atha yadapāmante krīṇāti | raso vā āpaḥ sarasamevaitatkrīṇātyatha yaddhiraṇyam bhavati saśukramevaitatkrīṇātyatha yadvāso bhavati satvacasamevaitatkrīṇātyatha yadajā bhavati satapasamevaitatkrīṇātyatha yaddhenurbhavati sāśiramevaitatkrīṇātyatha yanmithunau bhavataḥ samithunamevaitatkrīṇāti taṃ vai daśabhireva krīṇīyānnādaśabhirdaśākṣarā vai virāḍvairājaḥ somastasmāddaśabhireva krīṇīyānnādaśabhiḥ

Now the reason why he buys (the Soma) near water is that--water meaning sap--he thereby buys Soma sapful; and as to there being gold, he thereby buys him lustrous; and as to there being a cloth, he thereby buys him with his skin; and as to there being a she-goat, he thereby buys him fervid; and as to there being a milch cow, he thereby buys him with the milk to be mixed with him; and as to there being a pair (of kine), he thereby buys him with a mate.--He should buy him with ten (objects), and not with other than ten, for the virâg consists of ten syllables, and Soma is of virâg nature: therefore he should buy him with ten (objects) and not with other than ten.