Mundhir bin Abu Usayd was brought to the Prophet ﷺ when he was born, and the Prophet ﷺ placed him on his thigh while Abu Usayd sat nearby. The Prophet ﷺ became preoccupied with something in front of him, so Abu Usayd had his son taken away from the Prophet's ﷺ thigh.When the Prophet ﷺ turned his attention back, he asked, "Where is the boy?"Abu Usayd replied, "We took him away, Messenger of Allah."The Prophet ﷺ asked, "What is his name?"Abu Usayd answered, "So-and-so."The Prophet ﷺ said, "Rather, his name is Mundhir." So he named him Mundhir on that day.
أتي بالمنذر بن أبي أسيد إلى النبي ﷺ حين ولد، فوضعه على فخذه- وأبو أسيد جالس- فلها النبي ﷺ بشيء بين يديه، فأمر أبو أسيد بابنه، فاحتمل من فخذ النبي ﷺ فاستفاق النبي ﷺ، فقال: أين الصبي؟ فقال أبو أسيد: قلبناه يا رسول الله. قال: ما اسمه؟ قال: فلان قال: ولكن أسمه المنذر فسماه يومئذ المنذر.
Bukhari related it in al-Adab (6119), and Muslim in al-Adab (2149), both through the route of Saeed bin Abu Maryam, who said, Abu Ghassan (that is, Muhammad bin Mutarrif) told us, Abu Hazim told me, from Sahl bin Saad, and he related it.See, regarding choosing a good name for the newborn, Minna Kubra (4/539).