Shemoneh Esrei: Selach Lanu
The Mishna Berura writes that one should ponder one’s sins and wrongdoings when reciting the "selach lanu" blessing in shemoneh esrei. This is the source for the custom to gently beat one's chest while reciting the words "chatanu" and "pashanu" in the course of this blessing. There is also a Midrash which teaches that any time one recalls one’s sins, one should gently beat one’s chest. This is because banging on one's chest symbolizes and reminds us that forbidden desires originate in one's heart. It might just be that the custom to beat one's chest while reciting the words "chatanu" and "pashanu" during selach lanu is something that was adopted from the custom to do so when reciting "chatanu" and "pashanu" during the Yom Kippur prayers.
While it is nearly universal practice to beat one's chest during the selach lanu blessing, there are those who do not do so when reciting shemoneh esrei during a prayer service in which tachanun is not recited. This is because, according to some authorities, if tachanun is not recited then "vidui", confessions, are inappropriate, as well. As such, there are those who never beat their chest during the Shemoneh Esrei of ma'ariv, as tachanun is never recited at night. In a variation of this custom, there are those who don’t beat their chest if tachanun will not be recited after the shemoneh esrei, though they do beat their chest at every ma’ariv. In most communities, however, no distinction is made between whether or not tachanun will be recited after the shemoneh esrei. According to this approach, one beats one’s chest at every weekday shemoneh esrei without exception.
 Mishna Berura 115:1.
 Piskei Teshuvot 115:2.
 Kohelet Rabba 7:2.
 Magen Avraham 607:3.
 Mekor Chaim, cited in Piskei Teshuvot 115:1, Halichot Shlomo 11:45; Siddur Yaavetz, selach lanu.
 Chikrei Minhagim p.49.
 Shaarei Halacha U'minhag, OC 69.
 Piskei Teshuvot 115 footnote 11.