Source Wikipedia:
"According to the biblical story, God commands Abraham to offer his son Isaac as a sacrifice. (Genesis 22:5 and 22:8 ). After Isaac is bound to an altar, the angel of God stops Abraham at the last minute, saying "now I know you fear God." At this point Abraham sees a ram caught in some nearby bushes and sacrifices the ram instead of Isaac."
Muslims believe that this event happened to Ishmael rather than Isaac.
Just for info...