Sahih al-Bukhari 3910
صحيح البخاري#3910SahihNarrated 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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 63:135