Source and decoding - Rosh Hashanah 17b
בְּרִית כְּרוּתָה לִשְׁלֹשׁ עֶשְׂרֵה מִדּוֹת שֶׁאֵינָן חוֹזְרוֹת רֵיקָם, שֶׁנֶּאֱמַר: ״הִנֵּה אָנֹכִי כּוֹרֵת בְּרִית״.
Rav Yehudah said: a covenant was made with the thirteen attributes that they do not return empty, as it says "Behold, I make a covenant." The thirteen attributes are the attributes of mercy told to Moses after the golden calf, and they are the heart of the selichot prayers..
Rav Yehudah said: a covenant was made with the thirteen attributes that they do not return empty, as it says "Behold, I make a covenant." The thirteen attributes are the attributes of mercy told to Moses after the golden calf, and they are the heart of the selichot prayers.