Psalms 120:1

שִׁיר הַמַּעֲלוֹת אֶל ה’ בַּצָּרָתָה לִּי קָרָאתִי וַיַּעֲנֵנִי

A Song of Ascents. In my distress I called to G-d and He answered me.

At the conclusion of pesukei d’zimra, we recite the bracha of Yishtabach. This bracha contains fifteen expressions of praise to G-d, corresponding to the fifteen Psalms – starting with Psalm 120 – that begin with the words “shir hamaalos” – “a song of ascents.” The Talmud (Succah 54b) explains that these Psalms also correspond to the fifteen steps in the Temple. It was on these steps that the Levites would play their instruments and sings their songs of praise.