A chasid once forgot a sheaf in the field.  [It is a mitzvah to leave such sheaves in the field for the poor.]  The chasid was so happy that he told his son to offer two sacrifices in celebration.  When his son asked, “Why do you rejoice over this mitzvah more than any other?” the chasid replied: “God gave us all other mitzvoth to be done knowingly; this is the only one that must be done unknowingly” ~ Tosefta, Pe’ah 2