Canto 1 - Boyhood
Bāla-kāṇḍa
Chapter 39: Sagara’s Sons Search for the Sacrificial Horse
Text 1.39.7

तस्य पर्वणि तं यज्ञं यजमानस्य वासवः।
राक्षसीं तनुमास्थाय यज्ञीयाश्वमपाहरत्॥

tasya parvaṇi taṁ yajñaṁ yajamānasya vāsavaḥ
rākṣasīṁ
tanum āsthāya yajñīyāśvam apāharat

tasya parvaṇi = on the day of Ukthya; tam yajñam = that sacrifice; yajamānasya = belonging to the king who sponsored; vāsavaḥ = Indra; rākṣasīm tanum āsthāya = assuming the body of a rākṣasa; yajñīya-aśvam = the sacrificial horse; apāharat = stole.

On the day of Ukthya, assuming the body of a rākṣasa, Indra stole the sacrificial horse belonging to the king who sponsored that sacrifice.1

Indra assumed a rākṣasic body through his powers of delusion. Parvaṇi here indicates a particular day, as noted in the Nighaṇṭu.[2]

[2] tithi-bhede kṣaṇe parve.

1 Ukthya is a particular day within a sacrifice when a soma-yajña is conducted.