Imam al-Shafi'i (may Allah have mercy on him) said: "Of the women I have heard about, the earliest to menstruate are the women of Tihamah, who menstruate at the age of nine."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / Q15417
It is narrated from Abbad bin Abbad al-Muhallabi that he said: Among us, that is, the Muhallabis, I came across a woman who became a grandmother at the age of eighteen. She gave birth to a daughter at the age of nine, and her daughter also gave birth at the age of nine, so she became a grandmother at the age of eighteen.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15417
It is narrated from al-Layth (may Allah have mercy on him) that Abu Salih (may Allah have mercy on him) narrated to him from a man, who informed him that a daughter of his became pregnant when she was ten years old.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15418
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