Triangle ABC is reflected through a mirror. The mirror image hence obtained is again reflected through the same mirror. Then the final mirror image obtained is congruent to triangle ABC.
When any polygon is reflected through a mirror, its angles and the lengths of sides remain the same. i.e. it would retain the same size and shape.
Hence no matter how many times triangle ABC is reflected through the mirror, its mirror images will be congruent to it.
Therefore the given statement is true.