Whom

Often incorrectly used for who before he said or similar expressions, when it is really the subject of a following verb.

Poor: His brother, whom he said would send him the money.
Better: His brother, who he said would send him the money.

Poor: The man whom he thought was his friend.
Better: The man who (that) he thought was his friend (whom he thought his friend).

From: William Strunk, Jr., The Elements of Style, 1918.