Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: One night Sayyidah Umm Salamah (may Allah be pleased with her) was lying beside the Messenger of Allah (peace and blessings be upon him) when she suddenly got up as though trying to slip away unnoticed. He (peace and blessings be upon him) said: "What is the matter? Has your period begun?" She said: "Yes." He (peace and blessings be upon him) said: "There is no harm. Perform your ablution and come back to your place."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11552