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.