In the earliest scriptures Brahma/Prajapati/Hiranyagarbha is depicted as having no genealogy and sometimes equated with Parabrahmam.
for example:
lord-brahma-be-regarded-as-a-supreme-self:
but in later scriptures he is said to be born from a lotus sprouting from Vishnu's navel.
Durga Saptasathi, a part of Markandeya Purana, contains reference to Brahma, being emanated from the navel of Vishnu.
आस्तीर्य शेषमभजत्कल्पान्ते भगवान् प्रभुः। तदा द्वावसुरौ घोरौ विख्यातौ मधुकैटभौ॥६७॥
विष्णुकर्णमलोद्भूतो हन्तुं ब्रह्माणमुद्यतौ। स नाभिकमले विष्णोः स्थितो ब्रह्मा प्रजापतिः॥६८॥
दृष्ट्वा तावसुरौ चोग्रौ प्रसुप्तं च जनार्दनम्। तुष्टाव योगनिद्रां तामेकाग्रहृदयस्थितः॥६९॥
At the end of a kalpa when the universe was one ocean( with the waters of the deluge) and the adorable Lord Vishnu stretched out on Sesa and took the mystic slumber, tow terrible asuras, the well-known Madhu and Kaitabha, sprung into being from the dirt of Vishnu’s ears, sought to slay Brahma; Brahma, the father of beings, was sitting in the lotus( that came out) from Vishnu’s navel.
The oldest Shaastra would be the Rgveda, which indirectly mentions Brahmaa (Hiranyagarbha) taking birth from the navel of Vishnu.
तमिद्गर्भं॑ प्रथ॒मं द॑ध्र॒ आपो॒ यत्र॑ दे॒वाः स॒मग॑च्छन्त॒ विश्वे॑ ।
अ॒जस्य॒ नाभा॒वध्येक॒मर्पि॑तं॒ यस्मि॒न्विश्वा॑नि॒ भुव॑नानि त॒स्थुः ॥“The waters verily first retained the embryo (garbha) in which all the devas were aggregated, single deposited on the navel of the unborn (creator), in which all beings abide.”
-Rgveda, Mandala 10, Sukta 82, Mantra 6
Shripaada Madhvacharya quotes this mantra in his Brahma Sutra Bhaashya, too, as an example of Shri Vishnu being mentioned in the Rgveda. Other than this, the Puraanas, such as the Bhaagavata Puraana (Khanda 3, Adhyaaya 8) also describe the birth of Shri Brahmaa from the navel of Shri Vishnu.
Harihi Om.
Brahma is born of a cosmic egg and is not a navel-lotus-born offspring of Vishnu.
While he was lying there, taking repose in the Atman, he, the mighty Visnu, created a lotus in his umbilicus just for a sport.
Vayu Purana I.24.12
The lotus extended to a hundred Yojanas. It had the splendour of the midday-sun. It was very high supported by an adamantine stalk. It was created sportively by the powerful lord.
Vayu Purana I.24.13 .............
After saying thus, Visnu spoke again to Brahma, "O excellent Brahmana, you too, enter my belly and see the uncomparable world within."
Vayu Purana I.24.28
On hearing his words delightful to the mind and approving of them, Brahma entered the belly of Vishnu.
Vayu Purana I.24.29
Stationed within his body, he of unimaginable exploits, saw the same worlds. Though he wondered within he did not see the end of the worlds in the body of the primordial lord.
Vayu Purana I.24.30
On seeing that Brahma was returning, Visnu closed all the pores of his body. The lord then wished to resume his happy deep sleep in the middle of the vast ocean.
Vayu Purana I.24.31
When Brahma saw all the openings closed, he made his form very subtle and discovered a small opening in Visnu's umbilicus.
Vayu Purana I.24.32
Through the stalk of the lotus he came out and then resumed his own form. The four-faced lord seated in the middle of the lotus, shone with a splendour like that of the interior of the lotus.
Vayu Purana I.24.33
