«وَامْسَحُوا بِرُءُوسِكُمْ»
“And wipe your heads.”
Ba here features in a juristic debate. Translating it with a preposition does not alone determine exactly how much of the head must be wiped or how the action connects to its object.
Some scholars discuss a partitive reading, while the Hanafi legal-theory treatment retains attachment as the particle’s primary linguistic function. The full ruling on the extent of wiping depends on the relevant supporting evidence, not on isolating one particle from the rest of the law.