Imam Rida (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 al-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)
* FAZAILUL MAHDI (A.S.). Ali Akber Talafi. Translator : Syed Athar Hussein Rizvi. Al Qalam Translators & Writers Bureau