Shatapatha Brahmana 14.1.3.25

14.1.3.25

athemāmabhimṛśya japati manoraśvāsītyaśvā ha vā iyam bhūtvā manumuvāha so'syāḥ patiḥ prajāpatistenaivainametanmithunena priyeṇa dhāmnā samardhayati kṛtsnaṃ karoti

Thereupon, whilst touching her (the earth), 'Thou art Manu's mare,' for, having become a mare, she (the earth) indeed carried Manu, and he is her lord, Pragâpati: with that mate, his heart's delight, he thus supplies and completes him (Pragâpati, the Pravargya, and Sacrificer).