In this chapter of the Bhishma Parva of the Mahabharata, during one of the nights of the war Bhishma tries to convince Duryodhana to make peace with the Pandavas, arguing that they're invincible since they have Krishna on their side. To prove his point, Bhishma tells Duryodhana a hymn to Vishnu that was once said by Brahma:
Hear from me, O king, this hymn that was uttered by Brahman himself. This hymn was in days of old communicated by regenerate Rishis and the gods (to men) on Earth-
'Narada described thee as the Master and the Lord of the god of gods and all the Sadhyas and the celestials, and as one acquainted with the nature of the Creator of the worlds. Markandeya spoke of thee as the Past, the Present, and the Future, and the sacrifice of sacrifices, and the austerity of austerities. The illustrious Bhrigu said of thee that thou art the God of the gods, that thine is the ancient form of Vishnu. Dwaipayana said of thee that thou art Vasudeva of the Vasus, the establisher of Sakra, and the God of gods and all creatures. In days of old on the occasion of procreating creatures, the sages spoke of thee as Daksha, the Father of creation. Angiras said that thou art the creator of all beings. Devala said of thee that the unmanifest all is thy body, and the manifest is in thy mind, and that the gods are all the result of thy breath. With thy heads is pervaded the heavens, and thy two arms support the Earth. In thy stomach are three worlds and thou art the Eternal Being. Even thus do men exalted by asceticism know thee. Thou art the Sat of Sat, with Rishis gratified with sight of Self. With royal sages of liberal minds, never retreating from battle and having morality for their highest end, thou, O slayer of Madhu, art, the sole refuse.
Even thus is that illustrious and Supreme Being, viz., Hari, adored and worshipped by Sanatkumar and other ascetics endued with Yoga. The truth about Kesava, O sire, is now narrated to thee, both in brief and detail. Turn thy heart in love to Kesava.'
In the course of this hymn, the sages Narada, Markandeya, Bhrigu, Vyasa, Angiras, and Devala are all quoted by Brahma.
So my question is, where are these quotes taken from? Are these just things these sages said, or are they taken from works they're the seers or authors of? Some of these sages are seers of hymns of the Vedas; see the Rig Veda Anukramani compiled in my answer here. So could these quotes actually be Vedic verses?
On a side note, this isn't the only example of a hymn from Brahma addressed to Vishnu; see my answer here for another notable example.