How to quote a definition from a paper which cites another paper for the definition
In general, citing the original source is the way to go. As long as you cite correctly there is no need to worry about plagiarism.

Consequently, you should cite paper2. Since you cannot find the exact wording of the quote in paper1, I would suggest you paraphrase the quote and cite the original source with your own words. 

This would be much better than citing the already referenced quote in paper1.