She's too young
"She's too young" (إنها صغيرة, inna-haa sagheera) is a famous saying of Muhammad when he refused to marry his little daughter Fatima to Abu Bakr and Umar.
The sahih hadith can be found in the collection of an-Nasa'i in the chapter called "A woman marrying someone who is similar in age to her":
أَخْبَرَنَا الْحُسَيْنُ بْنُ حُرَيْثٍ، قَالَ حَدَّثَنَا الْفَضْلُ بْنُ مُوسَى، عَنِ الْحُسَيْنِ بْنِ وَاقِدٍ، عَنْ عَبْدِ اللَّهِ بْنِ بُرَيْدَةَ، عَنْ أَبِيهِ، قَالَ خَطَبَ أَبُو بَكْرٍ وَعُمَرُ رضى الله عنهما فَاطِمَةَ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِنَّهَا صَغِيرَةٌ " . فَخَطَبَهَا عَلِيٌّ فَزَوَّجَهَا مِنْهُ .
Narrated 'Abdullah bin Buraidah: It was narrated from 'Abdullah bin Buraidah that his father said: "Abu Bakr and 'Umar, may Allah be pleased with them, proposed marriage to Fatimah but the Messenger of Allah said: 'She is young.' (إِنَّهَا صَغِيرَةٌ, inna-haa sagheera) Then 'Ali proposed marriage to her and he married her to him."
صغيرة is the feminine version of صغير and it can mean young/small/tiny/little etc.  inna-haa sagheera (إنها صغيرة) is literally "Indeed-she [is] young/small"
The sahih hadith is also in Sahih ibn Hibban (6948) and others. 
Ali married Fatima shortly after the battle of Badr:
Narrated Husain bin `Ali:
`Ali bin Abi Talib said: "I got a she-camel as my share of the war booty on the day (of the battle) of Badr, and Allah's Messenger (ﷺ) gave me another she-camel. I let both of them kneel at the door of one of the Ansar, intending to carry Idhkhir on them to sell it and use its price for my wedding banquet on marrying Fatima.
According to Islamic tradition: