It is customary to recite during Minha on Shabbat a series of three verses, known as Sidkatecha. The three verses are:
1) "Sidkatecha Ke’harereh Kel Mishpatecha Tehom Rabba Adam U’behema Toshi’a Hashem."
2) "Ve’sidkatecha Elokim Ad Marom Asher Asita Gedolot Elokim Mi Kamocha."
3) "Sidkatecha Sedek Le’olam Ve’Toratecha Emet."
The reason why we recite these verses relates to a tradition that three great Sadikim died on Shabbat afternoon – Yosef, Moshe and David. The recitation of these verses serves as a "Siduk Ha’din," affirming God’s justice and declaring that we do not question His judgment, just as we recite at an Ashkaba.
There is some discussion among the commentators concerning the day of Moshe’s passing. Tosafot (commentaries to the Talmud by Medieval French and German scholars) note a comment in the Midrash that Moshe wrote thirteen Torah scrolls on the day he died. Obviously, it is forbidden to write on Shabbat, and the question thus arises as to how we can explain the tradition that Moshe died on Shabbat. The Bah (Bayit Hadash commentary by Rav Yoel Sirkis, Poland 1561-1640), in Orah Haim (292), answers that Moshe actually died on Ereb Shabbat, but he was buried on Shabbat. The soul ascends to its place in the heavens only when it is buried, and therefore we commemorate his death on Shabbat.
The Kaf Ha’haim (Rav Yaakob Haim Sofer, Baghdad-Israel, 1870-1939) cites the comment of the Ba’al Ha’Tanya (Rav Shneur Zalman of Liadi, 1745-1813) that one should preferably recite Sidkatecha while standing. Since this recitation is akin to an Ashkaba, is should be recited standing like an Ashkaba. This custom is also advocated by the work Mishbesot Zahab. Nevertheless, those who sit while reciting Sidkatecha have authorities on whom to rely.
When Shabbat falls on a day when Yehi Shem would be recited on a weekday, Sidkatecha is omitted.
Summary: It is customary to recite the three verses of "Sidkatecha" during Minha on Shabbat, except when Shabbat falls on a day when Yehi Shem would be recited during the week. One should preferably stand while reciting these verses.
