Hi Alberto,
For citing Ontop, you can refer to one of these two articles :
Guohui Xiao, Davide Lanti, Roman Kontchakov, Sarah Komla-Ebri, Elem Güzel-Kalayci, Linfang Ding, Julien Corman, Benjamin Cogrel, Diego Calvanese, and Elena Botoeva. The Virtual Knowledge Graph System Ontop. In: International Semantic Web Conference (Resource Track), 2020.
Diego Calvanese, Benjamin Cogrel, Sarah Komla-Ebri, Roman Kontchakov, Davide Lanti, Martin Rezk, Mariano Rodriguez-Muro, and Guohui Xiao. Ontop: Answering SPARQL Queries over Relational Databases . In: Semantic Web Journal 8.3 (2017), pp. 471–487.
Best,
Benjamin