A man asked the Messenger of Allah ﷺ about a man who has intercourse with his wife and then becomes lazy without ejaculating, whether she must perform the full bath. Ayesha was sitting nearby, and the Messenger of Allah ﷺ said, "I do that with this one, and then we both perform the full bath."
أن رجلا سأل رسول الله ﷺ عن الرجل يجامع أهله ثم يكسل، هل عليها الغسل؟ وعائشة جالسة، فقال رسول الله ﷺ: إني لأفعل ذلك أنا وهذه، ثم نغتسل.
Muslim (350) narrated it through Jabir bin Abdullah, from Umm Kulthum, from Ayesha, who mentioned the same thing. His ﷺ statement, "I do that, I and this one, then we both perform ghusl," shows that his ﷺ action indicates obligation, for otherwise it would not have provided an answer to the questioner. This is what Nawawi said.