Musnad Abu Ya'la al-Mawsili 42:78

Al-Qawariri narrated to us, Mu'adh bin Hisham al-Dastawa'i narrated to us, my father narrated to us, from 'Amir al-Ahwal, from Abu al-Siddiq, from Abu Sa'id al-Khudri, that the Prophet (peace and blessings of Allah be upon him) said: "Indeed, when the believer desires a child in Paradise, its age, its development, and its youth will be as he desires, or similar to it."