The first child born in Islam was Abdullah bin Zubayr. They brought him to the Prophet ﷺ, who took a date, chewed it softly, and then placed it into the baby's mouth. So the first thing to enter his stomach was the saliva of the Prophet ﷺ.
أول مولود ولد في الإسلام عبد الله بن الزبير، أتوا به النبي ﷺ، فأخذ النبي ﷺ تمرة فلاكها، ثم أدخلها في فيه، فأول ما دخل بطنه ريق النبي ﷺ.
Bukhari narrated it in the Virtues of the Ansar (3910), from Qutayba, from Abu Usama, from Hisham bin Urwa, from his father, from Ayesha, and mentioned it as related above.As for what Tirmidhi narrated (3826) from Abdullah bin Ishaq Jawhari, Abu Asim told us, from Abdullah bin Muammal, from Ibn Abu Mulayka, from Ayesha, that the Prophet ﷺ saw a lamp in the house of Zubayr and said, "Ayesha, I think Asma has gone into labor, so do not name him until I name him." So he named him Abdullah and rubbed a chewed date on his palate. This is daif.For Abdullah bin Muammal Makhzumi is daif in hadith. Ahmad said, "His hadiths are objectionable." Abu Hatim said, "He is not strong." The correct view is that he was born in Quba and was then brought to the Prophet ﷺ, as mentioned earlier. The house of Zubayr was close to the house of the Prophet ﷺ in Madinah.