asa(config)# crypto key generate rsa general-keys modulus 2048

a username:

asa(config)# username testuser password testpass

and the system should know where your useraccounts are:

asa(config)# aaa authentication ssh console LOCAL

Edit: And only allowing SSHv2:

asa(config)# ssh version 2

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https://supportforums.cisco.com/t5/firewalling/how-to-enable-ssh-on-asa-5525/td-p/2009812