Sahih Muslim
The Book of Destiny (Kitab-ul-Qadr)
Book Number: 033 / Hadith Number: 6391
This hadith has been reported on the authority of A'mash with the same chain of transmitters and in the hadith transmitted on the authority of Waki' (the words are):" The creation of any one of you is like this that (semen) is collected in the womb of the mother for forty nights," and in the hadith transmitted on the authority of Shu'ba (the words are):" Forty nights and forty days." And in the hadith transmitted on the authority of Jarir and 'Isa (the words are):" Forty days."