The Torah tells us: 

 “And the
children of Israel were in the wilderness, and they found a man gathering
sticks on the Sabbath day…” 

וַיִּהְיוּ בְנֵי־יִשְׂרָאֵל בַּמִּדְבָּר, וַיִּמְצְאוּ אִישׁ
מְקֹשֵׁשׁ עֵצִים, בְּיוֹם הַשַּׁבָּת (במדבר ט״ו:ל״ב) 

He is brought before Moshe. He is warned. And when he
refuses to stop, he is judged — and ultimately executed. 

 “And the
entire assembly removed him to the outside of the camp and stoned him with
stones, and he died, as Hashem had commanded Moshe.” 

וַיּוֹצִיאוּ אֹתוֹ כָּל־הָעֵדָה אֶל־מִחוּץ לַמַּחֲנֶה, וַיִּרְגְּמוּ
אֹתוֹ בָאֲבָנִים וַיָּמֹת, כַּאֲשֶׁר צִוָּה ה׳ אֶת־מֹשֶׁה (שם ט״ו:ל״ו) 

Immediately following this episode comes the mitzvah of
tzitzit: 

 “Speak to
the Children of Israel and say to them that they shall make for themselves
fringes on the corners of their garments…” 

דַּבֵּר אֶל־בְּנֵי יִשְׂרָאֵל וְאָמַרְתָּ אֲלֵהֶם, וְעָשׂוּ לָהֶם
צִיצִת עַל־כַּנְפֵי בִגְדֵיהֶם לְדֹרֹתָם… (שם ט״ו:ל״ח) 

Why this juxtaposition?