Комментарий к Йешайау 6:3

And one called to the other They would take permission from one another so that one would not commence before [his fellows] and be guilty of [a sin punishable by] burning, unless they all commenced simultaneously. This is what was established in [the blessing commencing:] “...Who formed light,” “the declaration of holiness, they all respond as one...” This is a Midrash Aggadah of the account of the Merkavah. And so did Jonathan render this.

And one cried unto another. The number of the seraphim is not mentioned; some believe they were two, but I am of opinion that there were many.

Holy, holy, holy Three times, as the Targum renders.