From Sahih Bukhari (Hadith 3910):
Narrated Aisha:The first child who was born in the Islamic Land (i.e. Medina) amongst the Emigrants, was `Abdullah bin Az-Zubair. They brought him to the Prophet. The Prophet (ﷺ) took a date, and after chewing it, put its juice in his mouth. So the first thing that went into the child's stomach, was the saliva of the Prophet
Reference: Bukhari 3910