You specify the interface in the following manner:

1. Define the server group and the protocol

2. Then specify the "source interface" and the host:

http://www.cisco.com/univercd/cc/td/doc/product/multisec/asa_sw/v_7_2/conf_gd/general/aaa.htm#wp1089730

hostname(config)# aaa-server AuthInbound protocol tacacs+

hostname(config-aaa-server-group)# max-failed-attempts 2

hostname(config-aaa-server-group)# reactivation-mode depletion deadtime 20

hostname(config-aaa-server-group)# exit

hostname(config)# aaa-server AuthInbound (inside) host 10.1.1.1

hostname(config-aaa-server-host)# key TACPlusUauthKey

hostname(config-aaa-server-host)# exit

HTH,

Chuck