Some creatures in D&D 3.5 have ability drain attacks, such as the Fang Dragon's bite attack. I remember reading somewhere that for such an attack to actually 'drain' it has to deal lethal damage, implying that for non hostile situations such a creature could use it's bite attack to inflict subdual damage and not permanently damage it's opponent.
The snippet below is taken from Draconomicon to illustrate the example's ability:
Ability Drain (Su): A fang dragon does not have a breath weapon, but its bite drains Constitution if the victim fails a Fortitude save. The number of Constitution points drained and the saving throw DC are given on the accompanying table.
The question is that I can't remember where I read that nor can I find it back now when looking for it. Does anyone know which particular book, rule or page covers this subject and if my 'assumption' is correct?