Is there any reference regarding this in any Veda or other books?
Who are Chiranjivis?
अश्वत्थामा बलिव्र्यासो हनूमांश्च विभीषण:। कृप: परशुरामश्च सप्तएतै चिरजीविन:॥ सप्तैतान् संस्मरेन्नित्यं मार्कण्डेयमथाष्टमम्। जीवेद्वर्षशतं सोपि सर्वव्याधिविवर्जित॥
8 Chiranjivis (immortals) are : Ashwatthama, King Bali, Krishna Dvaipāyana (Vedvayasa of 28th Dwapara Yuga), Hanuman, Vibhishan, Kripacharya, Parashurama and Markandaya.
Note: This list is highly disputed. Few sources only consider 7 Chiranjivis and few consider others as well (like Kak Bhusundi).
Ashwatthama, Krishna Dvaipāyana, Kripacharya and Parashurama
Will become one of the Saptrishis in 8th Manvantara. - Vishnu Purana
Note: Ashwatthama is also mentioned as Ved Vayasa in 29th Dwapara Yuga as well (Source). And Parashurama is Lord Vishnu's incarnation.
King Bali
Will hold the title of Indra (yes Indra is a post) in 8th Manavantara by the grace of Lord Vishnu. - Vishnu Purana
Hanuman
Considered as Lord Shiva's 11th Rudra incarnation.
Vibhishan and Markandaya
No information found, would love to hear comments.
Kak Bhusundi
Few folk stories say that Kakabhushundi has seen Ramayana 11 times with different outcomes and seen Mahabharat 16 times with different results, but, after seeing Daksha Yagya twice, he did not either care to see it again or saw no point to seeing any more, as it ended the same way each time.
