I think someone is confusing 2 or more scriptures.
In
Numbers 15:37-39, God does tell Moses to have the people sew more cloth on their clothes, but it has nothing to do with the length of the garments.
In
Matthew 23:5, Jesus speaks of the Pharisees taking
Numbers 15:38 to the extreme to bring attention to themselves and how religious they were, ignoring
Numbers 15:39 which says the ribbons are to remind the Israelites of who God is, not to remind the whole world, Jews and Gentiles alike, who the Pharisees were. This had nothing to do with a holiness/modesty standard, but was actually religiosity perverting holiness and making it immodest.
And forgive the digression, but to the naysayers: Yes, I am all too aware of this same trend among holiness ranks today. That doesnt mean that all holiness believers are "Neo-Pharisees", but all "Neo-Pharisees" are holiness believers and do take the holiness doctrine to the extreme while neglecting other, equally important doctrines altogether. So how does one detect the difference between a "Neo-Pharisees" and a true believer? Does the subject neglect other important doctrines and focus solely on holiness? Does the subject look down his nose at those with "lower" standards? Does the subject make the subject's standards all about the subject, rather than about God? If any of these questions are answered yes, the subject may be a "Neo-Pharisee" and should do some soul searching. If all of these questions are answered yes, this person
is a "Neo-Pharisee"
Now back on topic:
The closest thing to what you describe is found in
Exodus 28:42, but when you look at the context of
Exodus 28, you will understand this is not about correcting an immodest trend among the nation of Israel, but it is just one of a whole list of articles of clothing that is to be worn by the priests of the Levitical order:
(1)And take thou unto thee Aaron thy brother, and his sons with him, from among the children of Israel, that he may minister unto me in the priest's office, even Aaron, Nadab and Abihu, Eleazar and Ithamar, Aaron's sons.
(2)And thou shalt make holy garments for Aaron thy brother for glory and for beauty...
(4)And these are the garments which they shall make; a breastplate, and an ephod, and a robe, and a broidered coat, a mitre, and a girdle...
(5)And they shall take gold, and blue, and purple, and scarlet, and fine linen.
(6 )And they shall make the ephod of gold, of blue, and of purple, of scarlet, and fine twined linen, with cunning work.
verses 7-40 goes on to describe in great detail each of the aforementioned articles of clothing
(41) And thou shalt put them upon Aaron thy brother, and his sons with him; and shalt anoint them, and consecrate them, and sanctify them, that they may minister unto me in the priest's office.
(42) And thou shalt make them linen breeches to cover their nakedness; from the loins even unto the thighs they shall reach:
(43) And they shall be upon Aaron, and upon his sons, when they come in unto the tabernacle of the congregation, or when they come near unto the altar to minister in the holy place; that they bear not iniquity, and die: it shall be a statute for ever unto him and his seed after him.