Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 213
Narated By Mu'adh ibn Jabal : I asked the Apostle of Allah (pbuh): What is lawful for a man to do with his wife when she is menstruating? He replied: What is above the waist-wrapper, but it is better to abstain from it, too.
Abu Dawud said: This tradition is not strong.