The child’s gender comes from the man’s fluid and the woman’s fluid.

Although modern science discovered the answer much later, the Messenger of Allah ﷺ had already explained it.

As Allah says in the Qur'an:

“To Allah belongs the dominion of the heavens and the earth. He creates whatever He wills. He bestows female children to whom He wills and male children to whom He wills. Or He gives both males and females, and He renders barren whom He wills. Verily, He is All-Knowing, All-Powerful.”
(Ash-Shura: 49–50)

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The Prophet ﷺ said:

“Yes (the child resembles based on fluids). Then from whom comes the resemblance? Indeed, the man’s fluid is thick and white, and the woman’s fluid is thin and yellow. Whichever of them dominates or comes first, the child resembles that one.”
(Sahih Muslim 597)

Another Hadith:

Umm Sulaym asked the Messenger of Allah ﷺ about a woman who sees in a dream what a man sees (i.e., a wet dream). He ﷺ said:
“If she sees that and has a discharge, she must perform ghusl (ritual bath).”
Umm Salamah asked, “O Messenger of Allah, can that happen to a woman?”
He ﷺ replied: “Yes. If not, how would her child resemble her?”
(Sahih Bukhari 130, Sahih Muslim 313)

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Further Hadith Evidence:

Abdullah ibn Salam asked the Prophet ﷺ:

“What causes a child to resemble its father or mother?”
The Prophet ﷺ replied:
“If the man’s fluid comes first, the child resembles the father. If the woman’s fluid comes first, the child resembles the mother.”
(Sahih Bukhari 4480)

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Modern Scientific Viewpoint:
While Hadith explains resemblance and gender in terms of dominance and order of seminal fluids, modern genetics shows that the sperm from the male carries either an X (for girl) or Y (for boy) chromosome, and whichever fertilizes the egg determines the child’s sex. However, the Prophet ﷺ’s descriptions about the nature and influence of male and female fluids on the child’s characteristics align in spirit with what science later came to understand — that both parents contribute to physical and genetic traits, and the timing and conditions of fertilization may influence expression.