How do you know? Were you at the Last Supper?
And Christ didn't "tell us", He told his Apostles, his first priests.
What part of Mat 26:26 don't you understand? Are you saying the Eucharist was only for the Apostles?
While they were eating, Jesus took bread, and when he had given thanks, he broke it and gave it to his disciples, saying, "Take and eat; this is my body."