Sahih Muslim 2146 b
Asma' reported that she had become pregnant at Mecca with Abdullah b. Zubair (in her womt) and she (further) said:
I set out (for migration to Medina) as I was in the advanced stage of pregnancy. I came to Medina and got down at the place known as Quba' and gave birth to a child there. Then I came to Allah's Messenger (may peace he upon him). He placed him (the child) in his lap and then commanded for the dates to be brought. He chewed them and then put the saliva in his mouth. The first thing which went into his stomach was the saliva of Allah's Messenger (ﷺ). He then rubbed his palate with dates and then invoked blessings for him and blessed him. He was the first child who was born in Islam (after Migration).
حَدَّثَنَا أَبُو كُرَيْبٍ، مُحَمَّدُ بْنُ الْعَلاَءِ حَدَّثَنَا أَبُو أُسَامَةَ، عَنْ هِشَامٍ، عَنْ أَبِيهِ، عَنْ 
 أَسْمَاءَ، أَنَّهَا حَمَلَتْ بِعَبْدِ اللَّهِ بْنِ الزُّبَيْرِ بِمَكَّةَ قَالَتْ فَخَرَجْتُ وَأَنَا مُتِمٌّ، فَأَتَيْتُ الْمَدِينَةَ فَنَزَلْتُ
 بِقُبَاءٍ فَوَلَدْتُهُ بِقُبَاءٍ ثُمَّ أَتَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم فَوَضَعَهُ فِي حَجْرِهِ ثُمَّ دَعَا
 بِتَمْرَةٍ فَمَضَغَهَا ثُمَّ تَفَلَ فِي فِيهِ فَكَانَ أَوَّلَ شَىْءٍ دَخَلَ جَوْفَهُ رِيقُ رَسُولِ اللَّهِ صلى الله
 عليه وسلم ثُمَّ حَنَّكَهُ بِالتَّمْرَةِ ثُمَّ دَعَا لَهُ وَبَرَّكَ عَلَيْهِ وَكَانَ أَوَّلَ مَوْلُودٍ وُلِدَ فِي الإِسْلاَمِ ‏.‏