Tosefta su Isaia 6:14

In these Berachot (blessings) we bow down: the first Beracha (blessing) [of Shemoneh Esreh], in the beginning and at the end, and in [the paragraph of] Modim [in Shemoneh Esreh], in the beginning and at the end. And [as for] one who bows down in every [single] Beracha [of Shemoneh Esreh], we teach him that he should not bow down [in them]. We do not say [the Beracha] together with the one that says the Beracha [for you]. Rebbi Yehudah would say together with the one that made the Beracha, Kadosh Kadosh Kadosh Hashem Tzevaot Melo Kol Haaretz Kevodo (Isaiah 6:3) (Holy, Holy, Holy, Hashem Tzevaot, the earth is full with His glory) and Baruch Kevod Hashem Mimkomo (Ezekiel 3:12) (Blessed be the glory of Hashem from His place). All of these Rebbi Yehudah would say together with the one that made the Beracha.