Tashlich

Rama cites the custom of reciting the Tashlich (lit. "to cast") prayer by a body of water on Rosh Hashanah. This prayer asks God to cast away our sins. The Maharil explains that we recite it near water based on a Medrash that tells how When Avraham was on his way to Akeidas Yitzchak, the Satan created a river to block him and prevent him from fulfilling the Mitzvah. To remember the Akeidah, we recited the Tashlich prayer near water. The Ketzei Hamateh notes that this prayer may be said during the Ten Days of Repentance.