ssh -o "HostKeyAlgorithms ssh-rsa" user@hostname

ssh -Q key

HostKeyAlgorithms
             Specifies the protocol version 2 host key algorithms that the client wants to use in order of preference.  The default for this option is:

                ecdsa-sha2-nistp256-cert-v01@openssh.com,
                ecdsa-sha2-nistp384-cert-v01@openssh.com,
                ecdsa-sha2-nistp521-cert-v01@openssh.com,
                ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,
                ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,
                ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
                ssh-rsa,ssh-dss

             If hostkeys are known for the destination host then this default is modified to prefer their algorithms.