Śatapatha Brāhmaṇa ("of a hundred paths") 1.7.1.1

1.7.1.1

sa vai parṇaśākhayā vatsānapākaroti | tadyatparṇaśākhayā vatsānapākaroti yatra vai gāyatrī somamacāpatattadasyā āharantyā apādastābhyāyatya parṇam praciceda gāyatryai vā somasya vā rājñastatpatitvā parṇo 'bhavattasmātparṇo nāma tadyadevātra somasya nyaktaṃ tadihāpyasaditi tasmātparṇaśākhayā vatsānapākaroti

He (the Adhvaryu) drives the calves away (from the cows) with a parna branch. The reason why he drives the calves away with a parna branch is this. When the Gâyatrî flew towards Soma (the moon), a footless archer aiming at her while she was carrying him off, severed one of the feathers (parna) either of the Gâyatrî or of king Soma; and on falling down it became a parna (palâsa) tree; whence its name parna. May that which then was of the Soma nature be here with us now!' so he thinks, and for this reason he drives away the calves with a parna branch.