Al-Adab Al-Mufrad, Book 53, Hadith 1252 (English) ⓘ
Ibn Shihab said, "When a man became Muslim, he was ordered to have himself circumcised, even if he was old."
حَدَّثَنَا عَبْدُ الْعَزِيزِ بْنُ عَبْدِ اللهِ الأُوَيْسِيُّ قَالَ: حَدَّثَنِي سُلَيْمَانُ بْنُ بِلاَلٍ، عَنْ يُونُسَ، عَنِ ابْنِ شِهَابٍ قَالَ: وَكَانَ الرَّجُلُ إِذَا أَسْلَمَ أُمِرَ بِالِاخْتِتَانِ وَإِنْ كَانَ كَبِيرًا.
also referenced under: Hadith 1252 (Reference) & Book 53, Hadith 1252 (In-Book)