Bulugh al-Maram, Book 1, Hadith 141 (In-Book)

Narrated Abu Sa’id Al-Khudri (rad): Allah’s Messenger said: “If one of you has sexual intercourse with his wife and wishes to repeat he should perform ablution between them” [Reported by Muslim].

وَعَنْ أَبِي سَعِيدٍ اَلْخُدْرِيِّ ‏- رضى الله عنه ‏- قَالَ: قَالَ رَسُولُ اَللَّهِ ‏- صلى الله عليه وسلم ‏-{ إِذَا أَتَى أَحَدُكُمْ أَهْلَهُ, ثُمَّ أَرَادَ أَنْ يَعُودَ فَلْيَتَوَضَّأْ بَيْنَهُمَا وُضُوءًا } رَوَاهُ مُسْلِم ٌ (142)‏.‏ ‏3 ‏- صحيح.‏ رواه مسلم (308)‏.‏

sunnah.com

also referenced under: Book 1, Hadith 125 (English)