Combine sour cream, horseradish and chives. Season with salt and pepper.
Spread each slice of bread with butter. Spread 4 of the buttered slices with horseradish mixture. Top with roast beef. Top with remaining bread, buttered side down. Trim crusts from bread. Cut each sandwich diagonally into 2 triangles. (Sandwiches can be chilled, covered tightly with plastic wrap, for 1 hour before serving.)
