Sahih Muslim Book 26, Hadith Number 5404.

Chapter : It is excellent to tell a man when one is accompanied by one’s wife or Mahram lady that she is one’s wife or Mahram and to remove his doubt.

Anas reported that when Allah’s Messenger (may peace be upon him) was in the company of one of his wives a person happened to pass by them. He called him and when he came, he said to him: O so and so, she was my such and such wife. Thereupon he said, Allah’s Messenger, if I were to doubt at all, I would have entertained no doubt about you at least. Thereupon Allah’s Messenger (may peace be upon him) said: Verily Satan circulates in the body like blood.