"By our own tradition, the moisture at the Western Wall of the Holy Temple indicates the impending redemption of the people and land of Israel, the rebuilding of the Temple and the coming of Moshiach ben David, speedily and in our time, amen."
Gemara [Yoma 77b-78a] :
R' Pinchas said in the name of Rav Huna of Tzippori: The spring that issues from the Holy of Holies initially resembles the antennae of locust. Once it reaches the entrance of the Sanctuary it swells and becomes as wide as the thread of the warp. Once it reaches the entrance of the Antechamber, it becomes as wide as the thread of the woof. Once it reaches the entrance of the Courtyard it becomes as wide as the mouth of a small flask.
And this is the meaning of what we learned in the Mishna [Middos 2:6] R' Eliezer Ben Yaakov says: By this gate a stream of water, as wide as that which issues from a flask, will emerge from under the threshold of the Temple. From here onward [the stream] will rise unabatedly until it reaches the entrance of the house of David [i.e. Har Tzion, Mount Zion in Jerusalem - Rashi]. Once it reaches the house of David it becomes a swiftly flowing stream, in which zavin, zavos, niddos, and childbearing women will immerse and cleanse themselves from their tumah. As it is stated: On that day there will be a spring opened up for the house of David and for all the inhabitants of Jerusalem, for cleaning and for purification. [Zechariah 13:1]
And this is the meaning of what we learned in the Mishna [Middos 2:6] R' Eliezer Ben Yaakov says: By this gate a stream of water, as wide as that which issues from a flask, will emerge from under the threshold of the Temple. From here onward [the stream] will rise unabatedly until it reaches the entrance of the house of David [i.e. Har Tzion, Mount Zion in Jerusalem - Rashi]. Once it reaches the house of David it becomes a swiftly flowing stream, in which zavin, zavos, niddos, and childbearing women will immerse and cleanse themselves from their tumah. As it is stated: On that day there will be a spring opened up for the house of David and for all the inhabitants of Jerusalem, for cleaning and for purification. [Zechariah 13:1]