Ibrahim al-Nakha'i (may Allah have mercy on him) said: "The woman used to be commanded, when she prostrates, to press her stomach against her thighs so that her buttocks are not raised, and not to keep (her limbs) apart in the way a man keeps them apart."
Sayyiduna Ali (may Allah be pleased with him) says: When a woman prostrates, she should keep her thighs together.
Two weak hadiths have been narrated on this matter, the like of which are not used as evidence.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3197