Purification (Kitab Al-Taharah)
Dawud :: Book 1 : Hadith 213
Narrated Mu'adh ibn Jabal:
I asked the Apostle of Allah (peace_be_upon_him): 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.