Deuteronomy 33:21

וירא ראשית לו AND HE SAW THE FIRST PART FOR HIMSELF — i.e. he saw good to take for himself territory in the land of Sihon and Og, the subjugation of which was the beginning (ראשית) of the conquest of the Land, כי שם חלקת BECAUSE THERE WAS THE PORTION — i.e., because he knew that there in his territory was the portion of the field where was the grave of the מחקק, THE LAWGIVER, viz., Moses, ספון HIDDEN — i.e. that field was hidden and concealed from every creature, as it is said (Deuteronomy 34:6) “And no man knows his burial place” (Sifrei Devarim 355:6). ויתא AND HE CAME — i.e. Gad came. ראשי עם AT THE HEAD OF THE PEOPLE — They (the men of Gad) marched before the armed men at the conquest of the Land, because they were mighty, and thus does it state that Joshua said to them, (Joshua 1:14) “Ye shall pass armed before your brethren, etc.”. צדקת ה' עשה HE DID THE RIGHTEOUSNESS OF THE LORD — because they proved true to their word and kept their promise to cross the Jordan and to remain there until they (the Israelites) had conquered it and divided it amongst the tribes. — Another explanation: ויתא AND HE — the Lawgiver, Moses — CAME ראשי עם AT THE HEAD OF THE PEOPLE, צדקת ה' עשה HE DID THE RIGHTEOUSNESS OF THE LORD — This, too, is spoken of Moses (cf. Sifrei Devarim 355:9).

וַיַּ֤רְא רֵאשִׁית֙ ל֔וֹ כִּי־שָׁ֛ם חֶלְקַ֥ת מְחֹקֵ֖ק סָפ֑וּן וַיֵּתֵא֙ רָ֣אשֵׁי עָ֔ם צִדְקַ֤ת יְהֹוָה֙ עָשָׂ֔ה וּמִשְׁפָּטָ֖יו עִם־יִשְׂרָאֵֽל׃ {ס}        

And he provided the first part for himself, because there the portion of a lawgiver was reserved; and he came with the heads of the people, he executed the justice of the Lord, and his judgments with Yisra᾽el.