(45) باب هِجْرَةُ النَّبِيِّ صلى الله عليه وسلم وَأَصْحَابِهِ إِلَى الْمَدِينَةِ
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: Sahih al-Bukhari 3910
In-book reference: Book 63, Hadith 136
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 5, Book 58, Hadith 249