It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: Whoever puts his hand towards his private part and there is no cloth in between, then his ablution becomes obligatory again.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 8404
Hadith Gradingحکم دارالسلام:حديث حسن، وهذا إسناد ضعيف، يحيي ابن يزيد وأبوه ضعيفان، وهما متابعان