What is the earliest source for striking one's chest while reciting Vidduy?
The Encyclopedia of Jewish Prayer: The Ashkenazic and Sephardic Rites (Macy Nulman) has the following on the beating of the heart during Viduy
When saying Ashamnu we stand somewhat bent over, without leaning on any kind of support, just as in reciting Modim (MB, 607:10 ; Magen Avraham 607:4), a position of abject humility and contrition. One should beat upon his breast or heart with the fist at the mention of sin (Matt Eph 621:16). This practice is based on the verse vehahai yiten el libo, "And the living will lay it to his heart.". Rabbi Manna said, “Why does one beat the heart [in remorse for one’s sins]? This is to indicate that all [evil] comes from there “(Midrash Rabbah on Kohelet 7:2). Similarly, the Talmud states, “The heart and eyes are two agents of sin; the eye sees, the heart desires, and the instruments of action, that is, the hands, complete”. Hence, the hand beats the heart as if to denote that the heart caused wrongdoing (Jer. Ber. 36).