His last words were:

“No.”

Alexander Graham Bell was a Scottish inventor who worked in the United States for most of his life. While he is most famous for creating the telephone, Bell also held an appointment as a professor of vocal physiology at Boston University. There, he fell in love with and married one of his deaf students, Mabel Hubbard. After forty-five years of marriage, Bell was stricken with a fatal illness. As he lay dying, Mabel whispered to him, “Don’t leave me.” In response, Bell signed the word, “No.”
—From Last Words