Abu Huraira related that the Prophet of Allah (SAW) said: Whenever a man sits between the four limbs [shu’ab] of a woman and then exerts her [jahadaha], it is obligatory for him to wash.
-Sahih Muslim, The book of menstruation

Scholars have differed on the meaning of “the four limbs” [shu’ab]; it has been said that they are the two hands and the two legs, and it is also said the two legs and the two thighs, and it is also said the two legs and the two labia. Al-Qadi ‘Iyad opted for a meaning of the sides of the vagina and the sides of the forelocks.