Śatapatha Brāhmaṇa ("of a hundred paths") 3.2.1.29

3.2.1.29

tāṃ vā uttānāmiva badhnāti | uttāneva vai yonirgarbham bibhartyatha dakṣiṇām bhruvamuparyupari lalāṭamupaspṛśatīndrasya yonirasītīndrasya hyeṣā yonirato vā hyenām praviśanpraviśatyato vā jāyamāno jāyate tasmādāhendrasya yonirasīti

He ties it (to the end of the garment) with the open part upwards, for it is in this way that the womb bears the embryo. He then touches with it his forehead close over the right eyebrow, with the text, 'Thou art Indra's womb,'--for it is indeed Indra's womb, since in entering it he enters thereby, and in being born he is born therefrom: therefore he says, 'Thou art Indra's womb.'