(45) باب هِجْرَةُ النَّبِيِّ صلى الله عليه وسلم وَأَصْحَابِهِ إِلَى الْمَدِينَةِ
Narrated Asma:
That she conceived `Abdullah bin Az-Zubair. She added, "I migrated to Medina while I was at full term of pregnancy and alighted at Quba where I gave birth to him. Then I brought him to the Prophet (ﷺ) and put him in his lap. The Prophet (ﷺ) asked for a date, chewed it, and put some of its juice in the child's mouth. So, the first thing that entered the child's stomach was the saliva of Allah's Messenger (ﷺ). Then the Prophet rubbed the child's palate with a date and invoked for Allah's Blessings on him, and he was the first child born amongst the Emigrants in the Islamic Land (i.e. Medina).
Reference: Sahih al-Bukhari 3909
In-book reference: Book 63, Hadith 135
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 5, Book 58, Hadith 248