Very sure. It's not in the written rules of the game, but is a consequence of the rules. To place the opposing king in checkmate, you must have sufficient pie

Very sure. It's not in the written rules of the game, but is a consequence of the rules. To place the opposing king in checkmate, you must have sufficient pieces to threaten the square the king is on, and all adjacent squares, simultaneously. This means anywhere from 4-8 squares depending on the position. King and knight, alone, cannot threaten enough contiguous squares simultaneously to cause checkmate in any position, largely because the knight must place the opposing king in check (a king cannot get close enough), but due to the mechanics of the knight's move there will always be at least three adjacent squares not threatened by the knight at that moment, and at least one that cannot be threatened by the knight's king. You should feel free to try this yourself: place a king of one color on the board, and a king and knight of the other color, and then use only those pieces to try to construct a checkmate. It simply cannot be done. The best you can do is isolate the lone king in a corner and cause a draw by depriving it of moves (but it will not be in check at that time, so it will be stalemate, not checkmate). And as luiz-pv9 points out, king+knight is not the only "insufficient material" combination.