I am writing a paper. I have a problem and I want to prove that it is an NP-hard problem. However, for simplicity, I select a subset from my problem to prove that it is an NP-hard problem. Although I think that it is reasonable that if a subset of problem is NP-hard, it result in the problem being NP-hard, but I think writing this sentence in a paper need a reference. I will be thankful if anyone helps and gives me a reference for it.......................More explanations : My problem includes N boxs, B(1) through B(N). Every box B(i) has a size called t(i) and t(i)<=t(i+1). Now, to prove that my problem is NP-hard, I have considered a subset of my problem  such that there are arbitrary number of boxes with size t(N). And for this subset I have proved that my problem is NP-hard, as a result the problem is NP-hard. is this an acceptable assumption?
Relationship between an NP-hard problems with the subsets of them?
complexity theory;np hard