Midrasch zu Nechemja 8:5

(Fol. 39) Raba b. R. Huna said: "As soon as the scrolls are opened [in the synagogue to read the portion of the week], it is prohibited to speak even in Halacha, as it is said (Meh. 8, 5) And as he opened it, all the people became Amadu, and the word Amadu means silent, as it is said (Job. 32, 16) And should I wait longer, because they cannot speak, (Amadu) they stand still." R. Zera said: "We may infer that Amadu means silent from here (Neh. 8, 3) And the ears of the people were directed unto the book of the Law."