Just for reference…

Source port(s):

Switch# config t
Switch(config)# monitor session 1 source interface fastethernet 0/1
Switch(config)# monitor session 1 source interface fastethernet 0/2
Switch(config)# monitor session 1 source interface fastethernet 0/3
Switch(config)# monitor session 1 source interface fastethernet 0/4
(however many you want to monitor)

Destination port:

Switch(config)# monitor session 1 destination interface fastethernet 0/24

…then exit and:

Switch# show monitor session 1

It’ll show you what all ports are being monitored (sources) and what port to sniff on (destination). Fin.: