Negative commandment · Lo Ta’aseh #87 of 613
For the [High Priest's] breastplate not to come loose from the ephod
שלא ייזח החושן מעל האיפוד
The verse · Shemot 28:28
וְיִרְכְּסוּ אֶת־הַחֹשֶׁן מִטַּבְּעֹתָו אֶל־טַבְּעֹת הָאֵפוֹד בִּפְתִיל תְּכֵלֶת לִהְיוֹת עַל־חֵשֶׁב הָאֵפוֹד וְלֹא־יִזַּח הַחֹשֶׁן מֵעַל הָאֵפוֹד
“And they shall bind the breastplate by its rings to the rings of the efod with a lace of blue, that it may be above the finely wrought girdle of the efod, and that the breastplate be not loosed from the efod.”
The Rambam’s words
For the [High Priest's] breastplate not to come loose from the ephod, as [Exodus 28:28] states: "The breastplate shall not come loose from the ephod."
שלא ייזח החושן מעל האיפוד, שנאמר "ולא ייזח החושן, מעל האיפוד" (שמות כח,כח; שמות לט,כא).
Why · Sefer HaChinuch #100
That the breastplate not move from on top of the apron
“It is from the roots of the commandment that G-d, may He be blessed, wanted for our good to bring us merit by the aggrandizement of that holy house, [such that] everything in it be proper and set in its place.”
Who & when
“And this commandment that we not remove the breastplate from the apron is practiced at the time of the [Temple] by males and females.”
Where in the Mishneh Torah
Hilchot Vessels of the Sanctuary and Those Who Serve Therein הלכות כלי המקדש והעובדין בו