The Book of Manners and Etiquette
38/1. Chapter:
The Prohibition Of Taking The Kunyah Abul-Qasim, And The Names Which Are Recommended
Sahih Muslim
وَحَدَّثَنَا أَبُو كُرَيْبٍ، حَدَّثَنَا أَبُو مُعَاوِيَةَ، عَنِ الأَعْمَشِ، بِهَذَا الإِسْنَادِ وَقَالَ " إِنَّمَا جُعِلْتُ قَاسِمًا أَقْسِمُ بَيْنَكُمْ " .
This hadith has been reported on the authority of A'mash with the same chain of transmitters but there is a slight variation (of wording) that, instead of the word Bu'ithat (I have been sent), the word ju'ilat (I have been made) has been used.
Reference : Sahih Muslim 2133eIn-book reference : Book 38, Hadith 7USC-MSA web (English) reference : Book 25, Hadith 5320