ssh -i  identity-file(private-key)  

-i identity_file

Selects a file from which the identity (private key) for RSA or DSA authentication is read. The default is $HOME/.ssh/identity for protocol version 1, and $HOME/.ssh/id_rsa and $HOME/.ssh/id_dsa for protocol version 2. Identity files may also be specified on a per-host basis in the configuration file. It is possible to have multiple -i options (and multiple identities specified in configuration files).