How can we reconcile the Prophet's prohibition of sitting on graves with his action of sitting on his mother's grave?

Scholars have differed regarding the meaning of the Prophet's (peace be upon him) prohibition of sitting on graves. The Malikis and those who agreed with them held that the prohibition is against sitting on a grave to relieve oneself, while the majority understood the prohibition literally.

As for the phrase "he sat next to one of the graves," its meaning is that he sat beside it, not on top of it. The preposition "إلى" (ila) indicates proximity to a limit or goal, while "على" (ala) indicates being on top of something.

And Allah knows best.