TT #99 · Parashas Shoftim · Second Aliyah · Passuk 17:16
Only he must not multiply horses for himself, or cause the people to return to Egypt in order to multiply horses, since the Eternal has said to you, ‘You shall not return that way again.’
Torah Temimah points to Sifrei Devarim 158.
TT lemma: לֹא תֹסִפוּן לָשׁוּב.
English: ‘You shall not return that way again’ – this includes that they should not return to Egypt for any purpose.
Derash expanding the prohibition (Sifrei) — the ban is not limited to one specific motive.
Yisrael are forbidden to return to Egypt for any reason. The king may not create a situation that forces or encourages such a return.