Abu Basir narrated:
I told Abul Hasan (Musa bin Ja'far): "I sing my praises to God Who created you! How can Imam Mahdi (pbuh) be recognized?" He said: "There are several features by which Imam Mahdi (pbuh) is recognized. When people ask him questions, he answers them right away; if people are silent next to him, Mahdi (pbuh) begins speaking and tells what will happen tomorrow. He speaks with people in every language."
(Book of Belief and Blasphemy, Usul al-Kafi (Al-Usul min Al-Kafi), Al-Kulayni, Volume 1, page 402, 7/746)