Mishkat al-Masabih 3301

Sulaiman b. Yasar on the authority of Salama b. Sakhr, reported the Prophet (ﷺ) as saying about one who vows to make his wife like his mother’s back and has intercourse with her before making atonement, “There is one atonement.”*
* Atonement for breaking the vow should be made before doing so, but if the vow is broken then the same atonement is enough.
Tirmidhi and Ibn Majah transmitted it.