One of the most famous stories found in the Vedas is the story of Shunashepa, which is recited during the famous Rajasuya Yagna (the Vedic ritual for becoming an emperor). The story is told in this excerpt from the Aitareya Brahmana of the Rig Veda. There was once an ancient solar dynasty king, variously identified as Harishchandra or Ambarisha, who was childless, so on the sage Narada's advice he prayed to Varuna the ocean god. Varuna appeared before him and agreed to give him a son, on the condition that he would have to sacrifice the child to Varuna after he was born. Harishchandra agreed, and sure enough he soon got a son named Rohita. But when Varuna came for him, Harishchandra kept delaying giving up his son until his son grew up. Finally he could delay his promise no longer, and told his son that he had to be sacrificed. Rohita refused and ran away to the forest, where he met a starving Brahmin named Ajigarta. He offered him a hundred cattle for one of his sons. Out of desperation, Ajigarta gave up his son Shunashepa. Rohita returned to the kingdom with the boy, and Harishchandra prepared to sacrifice him to Varuna. But while tied up to the sacrificial post, Shunashepa prayed devoutly to the gods, reciting large number of Vedic mantras, and as a result his life was spared. Shunashepa was later adopted by the sage Vishwamitra, who was officiating the ritual.
But question isn't about the story itself, but rather the mantras that Shunashepa chanted. The Bala Kanda of the Ramayana describes two of the hymns he chanted:
When Shunashepa is tied to ritual post he immensely pleased two gods, namely Indra and Upendra as well, with those two hymns he got from Vishvamitra.
I think it's clear what the hymn to Indra is in reference to: as you can see in the Rig Veda Anukramani provided in my answer here, Shunashepa son of Ajigarta is listed as the seer of Rig Veda Book 1 Hymns 24-30. And several of those hymns, either in whole or in part, are addressed to Indra.
But my question is, what is the other hymn mentioned in the Bala Kanda, the one addressed to "Upendra"? For those who don't know, Upendra is a name of Vishnu, referring to the fact that in his incarnation as Vamana the dwarf he was was the little brother of Indra.
So where is Shunashepa's hymn to Vishnu? It's not mentioned in the excerpt from the Aitareya Brahmana I linked to above, and I don't see any hymns to Vishnu listed in the Rig Veda Anukramani for which Shunashepa son of Ajigarta was the seer. Does anyone know if it's in the Rig Veda or one of the other Vedas?