Mention by Panini
Adi Parva, Mahabharata/Book I Chapter 60 gives genealogy of all the principal creatures. Shishira (शिशिर) is mentioned in verse (I.60.20).... And Soma's son is the resplendent Varchas. And Varchas begot upon his wife Manohara three sons--Sisira, Parana and Ramana. ....