Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 212
Narated By Abdullah ibn Sa'd al-Ansari : Abdullah asked the Apostle of Allah (pbuh): What is lawful for me to do with my wife when she is menstruating? He replied: What is above the waist-wrapper is lawful for you.
The narrator also mentioned (the lawfulness of) eating with a woman in menstruation, and he transmitted the tradition in full.