I'm looking for first paper where was defined stringency (stringent condition) in hybridization of nucleic acids (for example for southern blot). Can you help me?
I am not sure; but I guess that has to be EM Southern's 1975 classic http://www.ncbi.nlm.nih.gov/pubmed/1195397.
For the reference to gradual optimization of stringency, you may want to peruse Southern's later articles listed below; these might provide you the clue you are looking for:
Measurement of DNA length by gel electrophoresis. I. Improved accuracy of mobility measurements using a digital microdensitometer and computer processing.