SB 6.9.52
sa va adhigato dadhyann
asvibhyam brahma niskalam
yad va asvasiro nama
tayor amaratam vyadhat
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
That saintly Dadhyañca, who is also known as Dadhici, personally assimilated the spiritual science and then delivered it to the Asvini-kumaras. It is said that Dadhyañca gave them mantras through the head of a horse. Therefore the mantras are called Asvasira. After obtaining the mantras of spiritual science from Dadhici, the Asvini-kumaras became jivan-mukta, liberated even in this life.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The following story is narrated by many acaryas in their commentaries:
nisamyatharvanam daksam pravargya-brahmavidyayoh; dadhyañcam samupagamya tam ucatur athasvinau; bhagavan dehi nau vidyam iti srutva sa cabravit; karmany avasthito ’dyaham pascad vaksyami gacchatam; tayor nirgatayor eva sakra agatya tam munim; uvaca bhisajor vidyam ma vadir asvinor mune; yadi mad-vakyam ullanghya bravisi sahasaiva te; siras-chindyam na sandeha ity uktva sa yayau harih; indre gate tathabhyetya nasatyav ucatur dvijam; tan-mukhad indra-gaditam srutva tav ucatuh punah; avam tava siras chittva purvam asvasya mastakam; sandhasyavas tato bruhi tena vidyam ca nau dvija; tasminn indrena sañchinne punah sandhaya mastakam; nijam te daksinam dattva gamisyavo yathagatam; etac chrutva tadovaca dadhyann atharvanas tayoh pravargyam brahma-vidyam ca sat-krto ’satya-sankitah.
The great saint Dadhici had perfect knowledge of how to perform fruitive activities, and he had advanced spiritual knowledge as well. Knowing this, the Asvini-kumaras once approached him and begged him to instruct them in spiritual science (brahma-vidya). Dadhici Muni replied, “I am now engaged in arranging sacrifices for fruitive activities. Come back some time later.” When the Asvini-kumaras left, Indra, the King of heaven, approached Dadhici and said, “My dear Muni, the Asvini-kumaras are only physicians. Please do not instruct them in spiritual science. If you impart the spiritual science to them despite my warning, I shall punish you by cutting off your head.” After warning Dadhici in this way, Indra returned to heaven. The Asvini-kumaras, who understood Indra’s desires, returned and begged Dadhici for brahma-vidya. When the great saint Dadhici informed them of Indra’s threat, the Asvini-kumaras replied, “Let us first cut off your head and replace it with the head of a horse. You can instruct brahma-vidya through the horse’s head, and when Indra returns and cuts off that head, we shall reward you and restore your original head.” Since Dadhici had promised to impart brahma-vidya to the Asvini-kumaras, he agreed to their proposal. Therefore, because Dadhici imparted brahma-vidya through the mouth of a horse, this brahma-vidya is also known as Asvasira.