In the Shanti Parva of the Mahabharata, Bhishma gives advice to Yudishthira and the Pandavas concerning how to be a good king and how to be good person, while he is lying on a bed of arrows after the end of the Kurukshetra war. In this chapter, Bhishma tells Yudhisthira about the duties of each of the four stages of life, namely Brahmachari (celibate student), Grihasthi (householder), Vanaprasthi (forest dweller), and Sanyasi (ascetic). Here is Bhishma says about the Grihasta stage of life:
The person leading the Garhasthya mode of life should, after studying the Vedas, accomplish all the religious acts laid down for him. He should beget children and enjoy pleasures and comforts. With careful attention he should accomplish all the duties of this mode of life that is applauded by ascetics and that is extremely difficult to go through (without transgressions). He should be satisfied with his own wedded wife and should never approach her except her season. He should observe the ordinances of the scriptures, should not be cunning and deceitful. He should be abstemious in diet, devoted to the gods, grateful, mild, destitute of cruelty, and forgiving. He should be of a tranquil heart, tractable and attentive in making offerings to the gods and the Pitris. He should always be hospitable to the Brahmanas. He should be without pride, and his charity should not be confined to any one sect. He should also be always devoted to the performance of the Vedic rites. In this connection, the illustrious and great Rishis cite a verse sung by Narayana himself, of grave import and endued with high ascetic merit. Listen to me as I repeat it.--'By truth, simplicity, worship of guests, acquisition of morality and profit, and enjoyment of one's own wedded wives, one should enjoy diverse kinds of happiness both here and hereafter.' The great Rishis have said that support of sons and wives, and study of the Vedas, form the duties of those that lead this high mode of life.
But my question isn't about these duties themselves, but rather the portion in bold, where Bhishma quotes a verse which was "sung by Narayana himself." My question is, where is this verse from? Is it a verse of the Vedas? And when Bhishma says that it was sung by Narayana, does he mean Vishnu himself or does he mean Vishnu's incarnation sage Narayana, whom I discuss here?
The fact that he says "endued with high ascetic merit" suggests that he's referring to the sage Narayana. And Narayana was the seer of the famous Purusha Sukta of the Rig Veda (see Book 10 Hymn 90 in the Rig Veda Anukramani here), so it's possible that this is another verse of the Vedas heard from the gods by Narayana.