Śatapatha Brāhmaṇa ("of a hundred paths") 4.3.5.14

4.3.5.14

yadveva dadhi gṛhṇāti | hutociṣṭā vā ete saṃsravā bhavanti nālamāhutyai tānevaitatpunarāpyāyayati tathālamāhutyai bhavanti tasmāddadhi gṛhṇāti

And the reason why he takes sour milk is that those remains (of Soma) poured together are the leavings of offerings, and insufficient for an oblation: he now increases those (remains), and thus they become sufficient for an oblation. This is why he takes sour milk.