The earliest panegyrics with Bhaktic content can be traced to hymns in the 7th Maṇḍala of the Ṛgveda in which Vasiṣṭa seeks reconciliation with Varuṇa, speaks of his contemplative mystic experiences, & contentious relationship with Varuṇa regarding moral ineligibilities.

Having concluded Varuṇa's role in establishing law & order in both the cosmic & socioethical senses & that Vasiṣṭa was instrumental in averting an inter-Vedic schism by reconciling both the Varuṇa & Iṇḍra cults, one can also note a domestic origin for Bhakti, not non-Indian

In RV 7.86.1 Vasiṣṭa greatly extols Varuṇa & demonstrates a substrate of 'proto-Bhaktic' constitution where the stanzas conceptualize the former as a Bhakta who longs a privileged communion desiring space within the innermost presence of Varuṇa despite a gulf separating them.

Some classical Bhaktic thematics in the RV 7.86-7.89 hymns are shown by Vasiṣṭa lamenting his moral ineligibilities to approach divinity (pṛcche tad enaḥ), non-volition of sins (acittiḥ), seeks redemption of his transgressions, complete surrender to Varuṇa's compassion...

...and by Vasiṣṭa considering himself a sakhā & constant kinsman of Varuṇa despite the former's lapses apropos Varuṇa (tvām āgāṁsi kṛṇvat sakhā te), & would serve Varuṇa as a slave (araṁ dāso na m ̄iḷhuṣe karāṇi), the dṛṣṭānta confirming the same w/bovid allegory.

Vasiṣṭa's orisons proffered unto Varuṇa yield for the former a vision of the latter (adhā nv asya saṁdṛśaṁ jaganvān RV 7.88.2) and Vasiṣṭa indeed uses this visualization to meditate upon Varuṇa.

Vasiṣṭa's mystic experiences buttress Bhaktic substratum hypotheses: Vasiṣṭa speaks w/hiraeth about their former associations, Varuṇa presents him the cosmic mystery of light & darkness & initiated the seer to the threefold seven names of the cow, & confers Ṛṣihood to him.