It was in Takshashila.
After Vaishampayana completed reciting the Mahabharata, Janamejaya is mentioned as returning to Hastinapura from Takshashila
The king honoured all the brahmanas and gratified them with dakshina. ‘They returned to wherever they had come from. Having taken his leave from the brahmanas, King Janamejaya left Takshashila and returned to Gajasahvya.
- 5, Swargarohana parva, Mahabharata.
Both Sarpasatra and narrating of the Mahabharata happened in Takshashila.