It is narrated from Sayyidah Aishah (may Allah be pleased with her) that Sayyidah Asma bint Abi Bakr (may Allah be pleased with her) came in while the Prophet (peace and blessings be upon him) was with Sayyidah Aishah (may Allah be pleased with her), and she was wearing thin Syrian clothes. He (peace and blessings be upon him) lowered his gaze to the ground and said: "O Asma! What is this? When a woman reaches maturity, it is not proper that anything of her be seen except such and such." And he (peace and blessings be upon him) pointed to his face and the palms of his hands.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13496