(a) Hisham bin Urwah narrates from his father that whoever touched his private part, ablution became obligatory upon him.
(b) Ibn Abi Mulaykah said: Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) was leading the people in prayer one day when his hand touched his private part. He signalled to the people to stay in their places, then he went and performed ablution, then came back and completed the rest of the prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 632