Do manual:
-I interface interface is either an address, or an interface name. If interface is an address, it sets source address to specified interface address. If interface in an interface name, it sets source inter‐ face to specified interface. For ping6, when doing ping to a link-local scope address, link specification (by the '%'-notation in destination, or by this option) is required.
Então, a resposta é:
ping -I eth1 123.123.123.123