It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "When a woman sits in the prayer, she should place one thigh over the other thigh, and when she prostrates, she should press her stomach against her thighs; this gives her the greatest covering. And when Allah Almighty looks at her, He says: 'O My angels! Bear witness that I have forgiven her.'"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3199