Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates that the Prophet (peace and blessings be upon him) said: "Whoever touches his private part should perform ablution."
Al-Bazzar and al-Tabarani (in al-Mu'jam al-Kabir) recorded it. The chain of al-Mu'jam al-Kabir contains a narrator, al-Ala' bin Sulayman, who is extremely da'if (weak), and the chain of al-Bazzar contains a narrator, Hashim bin Zayd, who is extremely da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1268