The Book of Manners and Etiquette
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
From Sahih Muslim, whose reports the scholars accept as authentic.