Payment method changed to manual – all subscriptions after migrating

HI

I was using a different stripe gateway plugin from webtoffee, and changed to your plugin, but now i find that all my subscriptions the payment method is changed to Manuel.
When i edit the subscription, payment method – and set it to Stripe, it is asking for :
Stripe Customer ID
Stripe Source ID

I know where to get the Stripe Customer ID from but not sure where in stripe the Stripe Source ID is — a screenshot will be appreciated.

Thank you