Arqam bin Shurahbil says: I was scratching my body during prayer and my hand touched my private part. I mentioned this to Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him), and he said, laughing: Cut it off! How can you separate it from yourself? It is a part of your body.
Al-Tabarani recorded it in al-Mu'jam al-Kabir, and its narrators have been declared trustworthy.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1260