Volume 3 · Book 48 · Hadith 826
Narrated Abu Said Al-Khudri: The Prophet said, "Isn't the witness of a woman equal to half of that of a man?" The women said, "Yes." He said, "This is because of the deficiency of a woman's mind."
— Sahih Bukhari, Volume 3, Book 48: Witnesses, Hadith #826