What was the only sign Jesus said he would give?

The sign of Jonah

An evil and adulterous generation seeketh after a sign; and there shall no sign be given to it, but the sign of the prophet Jonas:

For as Jonas was three days and three nights in the whale's belly; so shall the Son of man be three days and three nights in the heart of the earth. Matthew 12:39

(See also Matthew 16:4)