Śatapatha Brāhmaṇa ("of a hundred paths") 7.3.2.14

7.3.2.14

yadvevāghrāpayati | agnirdevebhya udakrāmatso 'paḥ prāviśatte devāḥ prajāpatimabruvaṃstvamimamanvica sa tubhyaṃ svāya pitra āvirbhaviṣyatīti tamaśvaḥ śuklo bhūtvānvaicattamadbhya upodāsṛptam puṣkaraparṇe viveda tamabhyavekṣāṃ cakre sa hainamuduvoṣa tasmāduvoṣa tasmādaśvaḥ śukla uduṣṭamukha ivāthā ha durakṣo bhāvukastamu vā ṛtveva hiṃsitveva mete taṃ hovāca varaṃ te dadāmīti

And, again, why he makes it smell; Agni went away from the gods; he entered the water. The gods said to Pragâpati, 'Go thou in search of him: to thee, his own father, he will reveal himself.' He became a white horse, and went in search of him. He found him on a lotus leaf, having crept forth from the water. He eyed him, and he (Agni) scorched him: hence the white horse has, as it were, a scorched mouth, and indeed is apt to become weak-eyed. He (Agni) thought he had hit and hurt him, and said to him, 'I grant thee a boon!'