Question : SQL select problem, if and only if using SQL 2005

I need to select contacts that have a subset of specific source codes (i.e. 'DFG20065', 'DS2456jsldf') but do not have any other records with different codes such as 'DTGHS35'.
For example,
select contactid, source from credhistory where
       source in ('DFG20065', 'DS2456JSLDF')
is my initial select.  However, I need to select contacts with these two and only these two source codes.
Can anyone help?
Thanks!

Answer : SQL select problem, if and only if using SQL 2005

select contactid, source from credhistory where
       source in ('DFG20065', 'DS2456JSLDF')
and not exists (select 1 from credhistory where  source not in ('DFG20065', 'DS2456JSLDF')  )