Sahih Bukhari Volume 4, Book 53, Hadith Number 345.

Narated By Jabir bin ‘Abdullah Al-Ansari : A man amongst us begot a boy whom he named Al-Qasim. On that the Ansar said, (to the man), “We will never call you Abu-al-Qasim and will never please you with this blessed title.” So, he went to the Prophet and said, “O Allah’s Apostle! I have begotten a boy whom I named Al-Qasim and the Ansar said, ‘We will never call you Abu-al-Qasim, nor will we please you with this title.’ ” The Prophet said, “The Ansar have done well. Name by my name, but do not name by my Kunya, for I am Qasim.”