Rashi on Deuteronomy 33:5/רש"י על דברים ל״ג:ה׳

1ויהי AND HE WAS — i.e. the Holy One, blessed be He, was

2בישרון מלך KING IN JESHURUN — constantly was the yoke of His sovereignty upon them at all times.

3בהתאסף When there gathered the head (בהתאסף ראשי), i.e., the total number (the whole) of their assembly.

4ראשי — The word ראשי has the same meaning as (Exodus 30:12) “When you take the count (רֹאשׁ) of the children of Israel”. Consequently they are worthy that I should bless them (and so I shall now begin the blessing: May Reuben live, etc.). — Another explanation of בהתאסף: When they assemble together in one band and there is peace between them, only then is He their King, but not when there is dissension among them (Sifrei Devarim 346:1).