Life as a woman under Isis: Document reveals for the first time what group really expects from female recruits living in Syria and Iraq per The Independent but the paper doesn't explain one aspect

The Independent reports on this ISIS document and includes this:

'Isis has released a manifesto detailing the role of women in the jihadist group - stating it is "legitimate" for girls to be married to fighters at the age of nine and emphasising their role as wives, mothers and homemakers.'

Why is it "legitimate" for girls to be married at the age of nine? What is the Islamic precedent for this?