The Gemara in Pesachim 112a says

A person should not drink water on Tuesday nights or [...] Friday nights. And if he drinks water, his blood is upon his own head, due to the danger. What is this danger? The danger of the evil spirit that rules on these days.

And if he is thirsty, what is his remedy? What should he drink? He should say the seven voices that David said over the water, and afterward he may drink. As it is stated:

קול ה' על המים אל הכבוד הרעים ה' על מים רבים קול ה' בכח קול ה' בהדר קול ה' שובר ארזים וישבר ה' את ארזי הלבנון קול ה' חוצב להבות אש קול ה' יחיל מדבר יחיל ה' מדבר קדש קול ה' יחולל אילות ויחשוף יערות ובהיכלו כולו אומר כבוד

We are all familiar with these verses because they are part of the Friday night prayer service (the last psalm before Lecha Dodi). So that explains why there is no problem in drinking water after prayer service on Friday nights. The gemara then brings other remedies if one doesn't say these verses to avoid the ruach ra: saying other verses, mentioning a friend, knocking the cover of the cup.

Two questions

  1. Is this psalm part of the prayer service because of this gemara?
  2. Is there any halacha that mentions precautions one should take before drinking water on Tuesday nights where the prayer service doesn't mention these verses? If not why not?
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  • BTW: The רשב"ם says it's only a problem באפילה - in the dark. I.e. don't drink water in the dark on those nights. (Source: toratemetfreeware.com/online/f_02340.html#HtmpReportNum0222_L2) – Danny Schoemann Jan 24 '18 at 15:17
  • @DannySchoemann thanks for this - but still should we take precautions on Tue night in the dark? if not why not? – mbloch Jan 24 '18 at 16:27
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    @DannySchoemann My girsa of Rashbam reads בלא נר, but I guess the meaning is the same. – DonielF Jan 24 '18 at 18:20
  • Re Q2 - How do you know that this is current halacha? There are a number of items in the Gemara similar to this idea that have not become current halacha, e.g. not drinking water that was left under your bed overnight. – DanF Jan 24 '18 at 19:09

Answer to Q1 - The recital of Tehillim 29 (הבו לה בני אלים) is because this Psalm alludes to the giving of the Torah which occurred on Shabbat (See Talmud Bavli Shabbat 76b.) I am summarizing the relevant points to this as mentioned in this beureihatefilah article:

Rada"k on the phrase קול ה' על המים explains that this is a reference to the giving of the Torah, which is compared to water.

See also Talmud Bavli Zevachim 116a that cites a Midrash that when the people felt the earth shaking, they asked Bilam what this was about. He said it is the giving of the Torah. Several verses from Tehillim 29 are cited, there.

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