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."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / Q3197