What's wrong in this scp statement?

Question

a=sys.argv[1]
b=sys.argv[2]
os.system("scp %s:/export/home/sample/backup.sql  %s:/home/rushi/abc.sql" % (a,b))

I'm accepting user name and machine name as the command line arguments. Like [email protected] This scp command is giving me following error:

[email protected]
[email protected]
Password:
Permission denied (publickey,keyboard-interactive).