Rashi on Numbers 15:39/רש"י על במדבר ט״ו:ל״ט

1וזכרתם את כל מצות ה׳ AND YE SHALL REMEMBER ALL THE COMMANDMENTS OF THE LORD — The ציצית will remind one of all the commandments because the numerical value of the letters of the word ציצית is six hundred, and there are eight threads and five knots in the fringes, so that you have six hundred and thirteen, which is also the number of the commandments of the Torah.

2ולא תתרו אחרי לבבכם — The verb has the same meaning as in (Numbers 13:25), “and they returned from searching (מתור) the land”. (The translation therefore is: AND YE SHALL NOT SEARCH AFTER YOUR OWN HEART). The heart and the eyes are the “spies” of the body — they act as its agents for sinning: the eye sees, the heart covets and the body commits the sin (Midrash Tanchuma, Sh'lach 15; cf. Talmud Yerushalmi Berakhot 1:8).