Qatadah narrates from al-Hasan (may Allah have mercy on him) that the Messenger of Allah (peace and blessings be upon him) said: "Allah Almighty does not accept the prayer of a woman who has reached puberty without a head-covering."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3255