Abu Sa'id al-Khawlani (may Allah have mercy on him) said: Ibn Wahb (may Allah have mercy on him) narrated to me; he said: al-Layth (may Allah have mercy on him) narrated to me; al-Layth (may Allah have mercy on him) said: My scribe Abdullah bin Salih (may Allah have mercy on him) narrated to me that a woman in their neighbourhood became pregnant when she was nine years old. And we have mentioned all the other accounts about this in the Book of Menstruation.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15419