Śatapatha Brāhmaṇa ("of a hundred paths") 2.6.3.7

2.6.3.7

tadyadvāyavyam bhavati | ayaṃ vai vāyuryo 'yam pavata eṣa vā idaṃ sarvam prapyāyayati yadidaṃ kiṃ ca varṣati vṛṣṭādoṣadhayo jāyanta oṣadhīrjagdhvāpaḥ pītvā tata etadadbhyo 'dhi payaḥ sambhavatyeṣa hi vā etajjanayati tasmādvāyavyam bhavati

Then as to why it is (offered) to Vâyu. Now Vâyu, indeed, is yonder blowing (wind); it is he that makes swell whatever rain falls here. But it is by the rain that plants grow; and on the plants being eaten and the water drunk, milk is produced out of that water. Hence it is he (Vâyu) that produces it; and for this reason it is (offered) to Vâyu.