It is narrated from Amr bin Shu'ayb (may Allah have mercy on him); he said: I was in the company of Sa'id bin al-Musayyib (may Allah have mercy on him) when those present raised the issue of touching the private part. Sa'id bin al-Musayyib (may Allah have mercy on him) said: "Busrah bint Safwan (may Allah be pleased with her), who is one of my maternal aunts, related: I was present in the gathering of the Messenger of Allah (peace and blessings be upon him), and so-and-so and so-and-so and Sayyiduna Abdullah bin Amr (may Allah be pleased with him) were also there, until she mentioned six or seven people. Then the Messenger of Allah (peace and blessings be upon him) said: 'Whoever touches his private part, let him perform ablution.'"
As for its first chain, it is sahih (authentic) and connected (muttasil), and this hadith of Busrah (may Allah be pleased with her) is in the four Sunan. Imam Ahmad (may Allah have mercy on him) also recorded it as the hadith of Sayyiduna Zayd bin Khalid (may Allah be pleased with him), but that is through the narration of Ibn Ishaq from al-Zuhri, from Urwah, from Zayd bin Khalid (may Allah be pleased with him). The chain we have given here makes it clear that Imam al-Zuhri did not hear this narration directly from Urwah, so it appears that Ibn Ishaq committed tadlis al-taswiyah in it, because he explicitly stated in that narration that he heard it from al-Zuhri. Therefore I have mentioned it through this route for the sake of scholarly benefit, while the remaining routes mentioned here were not recorded by the other hadith scholars.
Hadith Referenceالمطالب العالية بزوائد المسانيد الثمانية / كتاب الطهارة / 136Q
Hadith Gradingمحدثین:إسناده صحيح
Hadith Takhrij«حکم الحدیث المطالب العالیہ محققاً تنسیق د. سعد بن ناصر بن عبد العزيز الشَّثري- مکتبہ شاملہ :قال ابن حجر: أَمَّا الْإِسْنَادُ الْأَوَّلُ فَصَحِيحٌ مُتَّصِلٌ ، وَحَدِيثُ بُسْرَةَ فِي السُّنَنِ الْأَرْبَعَةِ ، وَأَخْرَجَهُ أَحْمَدُ مِنْ حَدِيثِ زَيْدِ بْنِ خَالِدٍ لَكِنَّهُ مِنْ رِوَايَةِ ابْنِ إِسْحَاقَ ، عَنِ الزُّهْرِيِّ ، عَنْ عُرْوَةَ ، عَنْ زَيْدِ بْنِ خَالِدٍ . » «أخرجه مالك فى «الموطأ» برقم: ، وابن الجارود فى "المنتقى"، 16، 17،، وابن خزيمة فى «صحيحه» برقم: 33، وابن حبان فى «صحيحه» برقم: 1112، والنسائي فى «المجتبیٰ» برقم: 163 ، والحاكم فى «مستدركه» برقم: 472، والترمذي فى «جامعه» برقم: 82، وابن ماجه فى «سننه» برقم: 479، والبيهقي فى«سننه الكبير» برقم: 623، والدارقطني فى «سننه» برقم: 527، والطيالسي فى «مسنده» برقم: 1762، والحميدي فى «مسنده» برقم: 355، والبزار فى «مسنده» برقم: 3762، وأورده ابن حجر فى "المطالب العالية"، 135/1، وأخرجه ابن أبى شيبة فى «مصنفه» برقم: 1735، وأخرجه الطبراني فى«الكبير» برقم: 5221، وأخرجه الطبراني فى «الأوسط» برقم: 480، وأخرجه الطبراني فى «الصغير» برقم: 1113»