Śatapatha Brāhmaṇa ("of a hundred paths") 7.4.1.13

7.4.1.13

yadveva puṣkaraparṇa upadadhāti | indro vṛtraṃ hatvā nāstṛṣīti manyamāno 'paḥ prāviśattā abravīdbibhemi vai puram me kuruteti sa yo 'pāṃ rasa āsīttamūrdhvaṃ samudauhaṃstāmasmai puramakurvaṃstadyadasmai puramakurvaṃstasmātpūṣkaram pūṣkaraṃ ha vai tatpuṣkaramityācakṣate paro 'kṣam paro 'kṣakāmā hi devāstadyatpuṣkaraparṇa upadadhāti yamevāsyaitamāpo rasaṃ samudauhanyāmasmai puramakurvaṃstasminnevainametatpratiṣṭhāpayati

And, again, why he puts it on the lotus-leaf. When Indra had smitten Vritra, he, thinking that he had not laid him low, entered the waters. He said to them, 'I am afraid: make ye a stronghold for me!' Now what essence of the waters there was that they gathered upwards (on the surface), and made it a stronghold for him; and because they made (kar) a stronghold (pûh) for him, therefore it is 'pûshkara;' 'pûshkara' being what is mystically called 'push-kara' (lotus-leaf), for the gods love the mystic. Now when he puts it (the gold plate) on the lotus-leaf, he then establishes him (Agni) in that essence which the waters gathered together for him (Indra), and in that stronghold which they made for him.