illegal substitution on the defense??

[MIcoach]

tonight we got this called on us and i am very confused as to why.

we had ten on the field defensively, at the snap, the 11th player comes on. the ref told us that he can not enter the field at the snap, it must be prior to.

in all my years of coaching i have never seen this penalty given to anyone.

i scoured the rulebook, but can not find in there were it says anything about substitution fouls. can someone enlighten me as to where it is, i would greatly appreciate it.

thanks