בנים אתם לה’ אלוקיכם לא תתגודדו ולא תשימו קרחה בין עיניכם למת. (דברים יד:א)
We are prohibited from cutting or balding ourselves over the death of someone close. Rashi explains that the introduction “Ye are the children of the Eternal” serves as a reason. It is not becoming of children of Hashem to maul themselves. Netziv points out that this explanation could have been gleaned from the next pasuk “כי עם קדוש אתה…”. It seems superfluous.
In addition to the pshat, the Gemara in Yevamot derives from this pasuk that we are not to splinter into different groups, לא תעשו אגודות אגודות.
The Gemara seems to take this Drasha seriously, as does Rambam (Avoda Zara 12;14). Rashi explains that “Agudot Agוdot” would lead observers to speculate that there are 2 Torahs chas v’shalom, wheras Rambam posits that the prohibition is to prevent Machloket”.
There are many details to this din, such as if it applies in different places and courts, and it gets especially interesting in times of Kibbutz Galuyot.
Netziv notes that the introduction of “Banim Atem” hints to the idea of “Agudot Agudot”. We are children of Hashem. Unity befits holy brethren.