Abu Qatan said that in the book it is recorded as a marfu hadith, "When he sits between her four limbs and exerts himself upon her, the bath becomes obligatory."
حدثنا عمرو بن الهيثم بن قطن وهو أبو قطن حدثنا هشام عن قتادة عن الحسن عن أبي رافع
أبو قطن قال في الكتاب مرفوع إذا جلس بين شعبها الأربع ثم جهدها فقد وجب الغسل
Its isnad is sahih. Hisham is Dastawai, and Abu Rafi is Saigh, Nufay bin Rafi. Bukhari narrated the hadith (1:337-338) through Muaz bin Fadala and Abu Naeem, both from Hisham, with this same isnad. Muslim narrated it (1:106) through Muaz bin Hisham Dastawai, from his father, from Qatada and Matar, from Hasan.As for the statement of Abu Qatan, "He said, in the book it is marfu," this is simply a quotation of Hisham Dastawai's own words. Hisham means by this to confirm and stress that the hadith is attributed to the Prophet, both from his memory and from his book.As for his saying, "between its four limbs," Ibn Athir said this refers to the two hands and two feet, though some said it means the two feet and the two labia, using this as a euphemism for intercourse. Ibn Daqiq Eid said in Sharh Umda (1:104-105) that in his view it is more likely to mean the two hands and two feet, or the two feet and two thighs, with intercourse being alluded to by this euphemism, the mention of these limbs being sufficient without need for explicit statement.As for his saying, "then he exerted himself upon her," Ibn Athir said this means he pushed and pressed her, as it is said, "the man exerted himself in the matter," when he strove hard and went to great lengths in it. Ibn Daqiq Eid said that this too is not meant literally, but rather what is intended by it is that ghusl becomes obligatory through intercourse even if no emission occurs.All of these are euphemisms, and grasping their meaning suffices without need for explicit statement.