Wikipedia article on Kusha says:
Kusha or Kusa (Sanskrit: कुश) and his twin brother Lava were the children of Lord Rama and Sita. Their story is recounted in the Hindu epic, the Ramayana. Hindu traditions claim he founded the city of Kasur in ancient times, though historical record shows the city was actually founded in 1525 by Pashtun migrants from what is now Afghanistan. His brother Lava is traditionally believed to have founded Lavapuri (current day city of Lahore).
Is there scriptural support for these claims?