Sahih Muslim 10567
In-book no 3290وَحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا وَكِيعٌ، عَنْ إِسْمَاعِيلَ، بِهَذَا الإِسْنَادِ قَالَ كُنَّا وَنَحْنُ شَبَابٌ فَقُلْنَا يَا رَسُولَ اللَّهِ أَلاَ نَسْتَخْصِي وَلَمْ يَقُلْ نَغْزُو .
This hadith has been narrated on the authority of Isma'il with the same chain of transmitters (and the words are): " We were young, so we said: Allah's Messenger, should we not have ourselves castrated? But he (the narrator) did not say; We were on an expedition."