Where does sage Vashishta describe Vishnu's immeasurable greatness?

Question

In the Srimad Bhagavatam, after describing Vishnu's incarnation (avataram) as Vamana the dwarf, it says this:

One who is subject to death cannot measure the glories of the Supreme Personality of Godhead, Trivikrama, Lord Viṣṇu, any more than he can count the number of atoms on the entire planet earth. No one, whether born already or destined to take birth, is able to do this. This has been sung by the great sage Vasiṣṭha.

That's the Prabhupada translation; here's the Aadhar translation:

The great saint [Vasishthha Muni] stated in his mantras that no mortal human being subject to death or destined to be reborn can measure the glories of Lord Urukrama, just as no one is capable of counting the number of atoms of the earth.

My question is, what exactly did Vashishta say, and in what scripture does he say it? The name Vashishta is not even in this verse; it just says "iti āha mantra-dṛg ṛṣiḥ puruṣasya yasya", meaning roughly "thus it was said about Purusha by a rishi who was a seer of mantras". (Purusha is a name for a form of Vishnu described in the Vedas.) All Prabhupada says in his commentary is this:

Vasiṣṭha Muni has given a mantra about Lord Viṣṇu: na te viṣṇor jāyamāno na jāto mahimnaḥ pāram anantam āpa. No one can estimate the extent of the uncommonly glorious activities of Lord Viṣṇu."

But where is this mantra from? I assume it's from one of the Vedas, because otherwise it wouldn't mention the fact that Vashishta is a seer of mantras; the Vedas are the collection of the mantras that was heard by seers (dhrishtas) from the gods. But where in the Vedas is this mantra found? My first thought would have been the Purusha Sukta, a hymn from the Rig Veda, since the verse refers to Purusha, but the seer of the Purusha Sukta is Vishnu's incarnation Narayana, not Vashista. (See book 10 hymn 90 in the Rig Veda Anukramani in this answer.). So it must be somewhere else.

Answer 1

The Anukramani of the Rig Veda says that Vashistha is the seer of the Seventh Mandala of the Rig Veda (see Book 7 in my answer here), and that is where I found the mantra heard by Vashishta. The reason I didn't find it initially is that Prabhupada writes it as "na te viṣṇor jāyamāno na jāto mahimnaḥ pāram anantam āpa", whereas the Rig Veda as it's presented here spells the beginning slightly differently: "na te viṣṇo jāyamāno na jāto deva mahimnaḥ param antam āpa".

In any case, here is the full hymn from the Rig Veda that is referenced by the Srimad Bhagavatam:

1 MEN come not nigh thy majesty who growest beyond all bound and measure with thy body. Both thy two regions of the earth, O Viṣṇu, we know: thou God, knowest the highest also.

2 None who is born or being born, God Viṣṇu, hath reached the utmost limit of thy grandeur. The vast high vault of heaven hast thou supported, and fixed earth's eastern pinnacle securely.

3 Rich in sweet food be ye, and rich in milch-kine, with fertile pastures, fain to do men service. Both these worlds, Viṣṇu, hast thou stayed asunder, and firmly fixed the earth with pegs around it.

4 Ye have made spacious room for sacrificing by generating Sūrya, Dawn, and Agni. O Heroes, ye have conquered in your battles even the bull-jawed Dāsa's wiles and magic.

5 Ye have destroyed, thou, Indra, and thou Viṣṇu, Śambara's nine-and-ninety fenced castles. Ye Twain smote down a hundred times a thousand resistless heroes of the royal Varcin.

6 This is the lofty hymn of praise, exalting the Lords of Mighty Stride, the strong and lofty. I laud you in the solemn synods, Viṣṇu: pour ye food on us in our camps, O Indra.

7 O Viṣṇu, unto thee my lips cry Vaṣaṭ! Let this mine offering, Sipivista, please thee. May these my songs of eulogy exalt thee. Preserve us evermore, ye Gods, with blessings.

Verse 2 is the specific mantra being discussed.