Imam Reza (A.S.) said:
"The most pious of believing men and women are those who are sorrowful for the (time of) flowing water (Imam Mahdi A.S)"
Flowing water indicates that it is a part of the ayat:
"Say ! Have you considered if your water should go down, who is if then that will bring you flowing water?"
(Sura Al Mulk 67 : 30)
According to Imam as-Sadiq (A.S.) "the flowing water" in this Ayat means Hazrat Mahdi (A.S.). Hence the exegesis of this Ayat is as follows:
"Say, if your Imam (A.S.) goes into ghaibat, who is it that will bring back your Imam to you. Who, like a gushing spring will teach you what is haraam and what is halal, and quench you thirst with good tidings (news) of the sky and the earth."
(Refer Kamaaluddin Pg.325-326)