The Talmud relates the following story:
Rabbi Joshua ben Levi met Elijah the Prophet standing by the entrance of Rabbi Simeon bar Yohai’s tomb…. He then asked him, “When will the Messiah come?”
“Go and ask him himself,” was his reply.
“Where is he sitting?”
“At the entrance.”
“And by what sign may I recognize him?”
“He is sitting among the poor lepers….”
So he went to him and greeted him, saying, “Peace upon you, Master and Teacher.”
“Peace upon you, O son of Levi,” he replied.
‘When will you come Master?” he asked.
“Today”, was his answer.
When he returned to Elijah, the latter enquired, “What did he say to you?”…
“He spoke falsely to me,” he said, “stating that he would come today, but he has not.”
Elijah answered him, “This is what he said to you: Today, if you will hear his voice.”
Babyonian Talmud: Sanhedrin 98a
(Based on the translation at Come and Hear.)