The First to Repent
Teshuvah
The Holy One, Blessed is He, said to Reuven that never had a person sinned before Him and repented unless he first faced punishment. Reuven was the first to repent, and so his descendant would be the first to speak of the greatness of teshuvah. This refers to Hoshea, who said, "Return, Yisroel, to Hashem your God" (Hoshea 14:2), teaching that repentance reaches up to the very Throne of Glory.
Source: Bereishis Rabbah 84:19