Sahih al-Bukhari 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.
حَدَّثَنَا قُتَيْبَةُ، عَنْ أَبِي أُسَامَةَ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ ـ رضى الله عنها ـ قَالَتْ أَوَّلُ مَوْلُودٍ وُلِدَ فِي الإِسْلاَمِ عَبْدُ اللَّهِ بْنُ الزُّبَيْرِ، أَتَوْا بِهِ النَّبِيَّ صلى الله عليه وسلم فَأَخَذَ النَّبِيُّ صلى الله عليه وسلم تَمْرَةً فَلاَكَهَا ثُمَّ أَدْخَلَهَا فِي فِيهِ، فَأَوَّلُ مَا دَخَلَ بَطْنَهُ رِيقُ النَّبِيِّ صلى الله عليه وسلم‏.‏