Rigveda 10.8, 2.11, 10.48, 10.99, 8.47 and Atharvaveda 6.113
Pan-IndiaAll traditions
Trita Aptya, in a cavern, knowing his father's weapons and urged by Indra, kills the three-headed, seven-rayed son of Tvashtri and frees his cattle; the next verse says Indra struck off the three heads. Elsewhere Indra hands Vishvarupa over to Trita, drives cattle from the dragon for him, and strengthens him to strike a boar with an iron-pointed shaft. In another hymn the Adityas and Dawn are asked to remove every evil deed and bad dream to far-off Trita Aptya, as a debt is paid off. An Atharvan charm says the gods wiped sin on Trita and he on mankind, and that it lies in twelve places.
Agni, pulled by force from the waters where he hid, spits on them as an unsafe refuge; from this the Aptya gods Trita, Dvita and Ekata arise. They go about with Indra like a Brahman in a king's train. When three-headed Vishvarupa is to be killed they know of it, and Trita kills him; Indra, being a god, is free of the sin. People say those who knew should bear it and the sacrifice should wipe it on them, so the rinsing water is poured out for the Aptyas. They pass it on to whoever sacrifices without a gift to the priests, and so a gift of rice is ordained. The water is warmed and poured separately for each, by name.
What is different here:
Gives Trita a birth and two brothers; makes Trita alone the killer; explains the transfer of guilt and a detail of ritual (the rinsing water, the dakshina).
Sources: Hymns of the Rigveda, tr. R. T. H. Griffith (1896), hymns read for the Vedic layer; Hymns of the Atharva-Veda, tr. R. T. H. Griffith (1895); The Satapatha-Brahmana, tr. Julius Eggeling (Sacred Books of the East 12, 41)