Sahih al-Bukhari, Book 67, Hadith 9 (In-Book) ⓘ
Narrated Ibn Masud: We used to fight in the holy battles in the company of the Prophet and we had no wives with us. So we said, "O Allah's Messenger! Shall we get castrated?" The Prophet forbade us to do so.
حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، حَدَّثَنَا يَحْيَى، حَدَّثَنَا إِسْمَاعِيلُ، قَالَ حَدَّثَنِي قَيْسٌ، عَنِ ابْنِ مَسْعُودٍ ـ رضى الله عنه ـ قَالَ كُنَّا نَغْزُو مَعَ النَّبِيِّ صلى الله عليه وسلم وَلَيْسَ لَنَا نِسَاءٌ فَقُلْنَا يَا رَسُولَ اللَّهِ أَلاَ نَسْتَخْصِي فَنَهَانَا عَنْ ذَلِكَ.
also referenced under: Hadith 5071 (DarusSalam) & Volume 7, Book 62, Hadith 9 (USC-MSA)