Sayyidah Asma' (may Allah be pleased with her) said: I was pregnant with Sayyiduna Abdullah bin al-Zubayr (may Allah be pleased with them both) in Makkah. When I set out (for the emigration), I was at full term. I reached Quba' in Madinah and gave birth to him there. Then I came to the Messenger of Allah (peace and blessings be upon him). He (peace and blessings be upon him) placed him in his lap, then called for a date, chewed it and put it in the mouth of Sayyiduna Abdullah (may Allah be pleased with him), and also put in his own saliva. So the first thing to enter the stomach of Sayyiduna Abdullah (may Allah be pleased with him) was the saliva of the Messenger of Allah (peace and blessings be upon him). Then he (peace and blessings be upon him) supplicated for him and prayed for blessing for him, and he was the first child born in Islam.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب اللقطة / 12147