Configuring Routers as NTP servers and ASA as client

On the router:

Router(config)# ntp master 2

! Sets the router as an ntp server with stratum 2

Router(config)# ntp authenticate

Router(config)# ntp authentication key 3 md5 ntpkey

Router(config)# ntp trusted key 3

! Sets up ntp authentication defining a key with index 3 and then selecting it as trusted

On the ASA:

ASA(config)# ntp authentication-key 3 md5 ntpkey

ASA(config)# ntp authenticate

ASA(config)# ntp server 10.1.1.240 key 3