I would like to know please, what is the source in the Tora of the concept of "Asseret Yeme Techouva" ? It seems to be a simple question, but I don't find any accurate answer.
Thank you very much for your help.
Gmar Hatima Tova.
'Aseres Yemei Teshuvah' is a Rabbinical institution, so no direct Torah source for it is available.
However, like most important Rabbinical institutions, one would expect to find a hint in the Torah, or at least somewhere in T'nach. Sure enough, the Gemara in Rosh Hashanah (18a) refers to such a hint.
The Pasuk in Yeshayah "Seek Hash-m when He is to be found" (implying that there is a specific period during the year when Hash-m is more open to Teshuvah), says the Gemara, refers to the Aseres Yemei Teshuvah, when the Teshuvah of every individual is acceptable to Hash-m.
Then the Gemara cites the Pasuk in Shmuel "And it was in ten days time, Hash-m plagued Naval (ha'Karmeli, and he died)". Those ten days, says the Gemara, refers to the Aseres Yemei Teshuvah. In other words, the incident with David Hamelech recorded in the Pasuk there took place on Rosh Hashanah, and Hash-m gave Naval ten days during which to repent (as Rashi explains). When he failed to do so, Hash-m took his life.
be'Virchas Kol Tuv