Musar for Taanit 43:19

In Sefer Charedim we find that even though one is obligated in all of the mitzvoth he must still hold fast to one of them, with great strength and constancy, that he does not transgress all the days of his life. For the generality of Torah, comprised of the 613 mitzvoth, is called a tree of life, viz. (Mishlei 3:18): "It is a tree of life to those who hold fast to it." And one who holds fast to one branch of a tree holds all of them, all being one body, following after that branch. But if he tried to hold on to all of the branches together, he could not do so. And this is the intent of Shabbath 118b: "R. Nachman b. Yitzchak said: 'May it [(reward)] so come to me that I fulfilled [the mitzvah of] the three Sabbath meals.' …R. Sheheth said: 'May it so come to me that I fulfilled the mitzvah of tzitzith!' R. Yosef asked R. Yosef b. Rabbah: 'Which mitzvah is your father most scrupulous in?' He answered: 'The mitzvah of tzitzith. One day he was going up a ladder when one of his fringes tore, and he would not come down until he had repaired it.'" And Rashi explains "I fulfilled the mitzvah of tefillin" as his not having walked four ells without tefillin. Similarly, with tzitzith… See the Sefer Charedim there, who adduces proof for his words from the Yerushalmi, too, that because of this, one's days will be enhanced and lengthened in the world which is entirely good and long.