My guess is this sample isn't from another song at all. It's the sample ATB used in almost all his remixes he did at the time. His basslines and this sample were just about his 'trademark' then.

Just check out some of his other 'own' stuff ('like 9PM - Till I come', 'Don't stop' or 'The Summer') and I'm sure you'll recognise the sample. It's not the same melody, but it's the same 'instrument'.