עזרא ג:י פירוש: רש״י

And...laid the foundation וְיִסְדוּ This is the strong conjugation of יְסוֹד, like (Ezek. 25:4): “and they will settle (וְיִשְּׁבוּ) their palaces.” Just as וְיִשְּׁבוּ is derived from ישּׁב so is וְיִסְדוּ derived from יסד, and their meaning is, וְיִסְדוּ וְיִשְּׁבוּ in the intensive conjugation with two “yud”s, one for the [continuous] action and one for the radical, and their meaning is that they were laying the foundation, they were settling.

And the builders laid the foundation And the builders were laying the foundation [see above] of the Temple of the Lord at the time [that] they were harmonizing in their song, and they stationed the priests attired with elegant garments with trumpets in their hands to sound them.

and the Levites the sons of Asaph were the singers.