Shatapatha Brahmana 9.1.1.39

9.1.1.39

yadvevāha daśa-daśeti | daśa vā añjaleraṅgulayo diśi-diśyevaibhya etadañjaliṃ karoti tasmādu haitadbhīto 'ñjaliṃ karoti tebhyo namo astviti tebhya eva namaskaroti te no mṛḍayantviti ta evāsmai mṛḍayanti te yaṃ dviṣmo yaśca no dveṣṭi tameṣāṃ jambhe dadhma iti yameva dveṣṭi yaścainaṃ dveṣṭi tameṣāṃ jambhe dadhātyamumeṣāṃ jambhe dadhāmīti ha brūyādyaṃ dviṣyāttato 'ha tasminna punarastyapi tannādriyeta svayaṃnirdiṣṭo hyeva sa yamevaṃviddveṣṭi

And as to why he says 'ten-ten,'--there are ten fingers in the joined hands: he thus makes reverence to them in each direction; and hence he who is frightened places his hands together;--'To them be reverence!'--he thus does reverence to them;--'May they be gracious unto us!'--they thus are gracious unto him;--'whomsoever we hate, and whosoever hateth us, him we cast into their jaws!'--thus he casts into their jaws whomsoever he hates, and whosoever hates him. He may say, 'So-and-so I cast into their jaws!' naming him whom he hates, and then he has no longer any hold upon him. But let him take no notice of this (injunction), for indicated of himself is he whom he who knows this hates.