Musnad Ahmad 31:77

Ali ibn Abdullah narrated to us, Mu'adh ibn Hisham narrated to us, he said: My father narrated to me, from 'Amir al-Ahwal, from Abu al-Siddiq, from Abu Sa'id al-Khudri, that the Prophet of Allah, peace and blessings of Allah be upon him, said: "If the believer desires a child in Paradise, its conception, its delivery, and its age will occur in a single hour, just as he desires."