Sayyidah Aishah (may Allah be pleased with her) narrates from the Prophet (peace and blessings be upon him) that the Messenger of Allah (peace and blessings be upon him) was asked: What is permissible for a man when his wife is menstruating? He (peace and blessings be upon him) said: "Whatever is above the lower garment (izar)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14082