The child on the banyan leaf (Markandeya's vision)
Markandeya asks to see Vishnu's maya and finds himself alone in the world-flood. On a banyan he sees a child on a leaf sucking his toe, is breathed into the child's body and sees the whole world there, then wakes at his hermitage. Prayag's undying banyan, the Akshayavat beside the Kumbh confluence, is told as that tree.
Markandeya, whose austerities on the Pushpabhadra please Nara and Narayana, asks to see Vishnu's maya. One evening a storm and flood engulf the world, and he wanders alone in the waters for what seems ages. He sees a banyan, and on a leaf a radiant infant sucking his toe. Drawn in by the child's breath, he sees the whole universe, including himself, within the child's body; breathed out, he reaches for the child, and finds himself back at his hermitage, the vision over.
Markandeya tells the Pandavas how, at the end of an age, he swam alone in the waters and saw a boy on a bed on a banyan branch. The boy invites him to rest in his body; Markandeya enters his mouth and wanders the whole world within for more than a hundred years without reaching its end. Coming out, he asks who the boy is, and is told he is Narayana, who sleeps on the waters between ages. Markandeya says the boy is Krishna.
What is different here:
Told by Markandeya to Yudhishthira; he enters by the mouth; the boy names himself Narayana.
Sources: Bhagavata Purana 12.8 to 12.9, Markandeya's vision (English translation, Vedabase); Mahabharata, Vana Parva 186 to 187 (Markandeya-Samasya; K. M. Ganguli translation, sacred-texts)