Which did you check? Here is Gerou and Lusk.
As it happens I think that best practice in this specific case is for the clef to transpose, not the singer (unlike piccolo where the player transposes and not the clef), but the situation is not black and white.
Even Gould, who is usually the most opinionated of authors, only states the use of the clef with an 8 for tenor singers is to be recommended (not compulsory).
