Mus'ab bin Sa'd narrated: I used to hold the mushaf for Sayyiduna Sa'd bin Abi Waqqas (may Allah be pleased with him), and (one day) I scratched myself. Sayyiduna Sa'd (may Allah be pleased with him) said: Perhaps you touched your private part? I said: Yes! He said: Get up and perform ablution. So I got up, performed ablution, then came back.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 411