Which book is being referred to here?

Matthew 24:15 When ye therefore shall see the abomination of desolation, spoken of by Daniel the prophet, stand in the holy place, (whoso readeth, let him understand:)

Those words which the Holy Ghost led to be placed there, shown here within the parentheses, those words, "whoso readeth, let him understand",

are they a reference to those who read that part of Daniel, or to those who read this part of Matthew, or is it both?