Narrated Sa'ed Al-Koudary: The prophet was asked about" Alazal"(An ancient method of preventing conception by letting sperm spilled out), he said:

"Not from all the sperm a fetus is created and if God wills to create anything, nothing can debar him" (Sahih Muslim, marriage chapter, 1438)

This Hadith gives a scientific statement that is really a striking scientific miracle because at the time of the prophet no one could dare to think that a fetus is created from a very little amount of sperm. No one, even till a few decades ago could think that an amount of ejaculate may contain between million to 600 million and that only one spermatozoonis needed to fertilize the ovum. 

How many sperms do an ovum need to be fertilized ?

Dr Lezy, in his book" Developmental Anatomy", says

" The conducted experiments on mammals have proven that only (1%) of sperm is sufficient to fertilize an ovum, it is medically established that (20 million and even less) of seamen in one ejaculate is the minimum quantity to fertilize an ovum. A less amount can inseminate an ovum in" Insemination Artificial".

Back again to the Hadith

The prophet's Hadith clearly says that not all the sperm is needed in insemination, just a little amount is needed, how could a man living in the desert fourteen years ago could know this scientific fact unless he was a true prophet, there is a another Quranic verse that stress the same fact,

God says:

Then He made his posterity out of the " Sulala" of a liquid disdained. 

The Arabic word" Soulala" has two meanings: the first is" carefully selected" and the other is" fish", both meanings are correct when talking about the seamen.

The significance of the Arabic word" Sulala"

 It is scientifically established that   a large number of spermatozoon die in vagina, a another number die at the neck of the uterus, about( 500) seamen reach the ovum and only one of them inseminates the ovum, this one is always  the most suitable and the strongest, this scientific fact supports the Quranic word"Soulala" which means the carefully selected or the best thing.

It is known that a seaman resembles" a fish", both have a head and a tail, the motion of each while swimming is identical.

Please note that at the time of the Prophet Muhammad, there were no microscopes. So it is easy to understand the significance of the Quranic word when describing the human spermatozoon, the Arabic word means:

The best selected thing, we already discussed that the ovum is fertilized by the strongest Spermatozoon.

It also means " fish", the spermatozoon  physically looks much like a fish.

Did Muhammad have an electronic microscope? The plain answer is " No"

The other part of the Hadith says

": If God wills something, nothing could debar Him."

This part reflects another miracle because it is known that all means of conception, no matter what is, are not always successful, and many pregnancies happen despite the existence of the means of conception.
 

Even surgeries for preventing pregnancy are not always fruitful; a woman came to me complaining that she was pregnant though she had tied-tube surgery. Hysterectomy is medically reported that it is not always fruitful as in this case (55%) of these surgeries fail if done via vagina and the chances are fewer if done via belly, so nothing could debars what God wills to do as stated in the Hadith.