Sahih Muslim 12723
In-book no 5446وَحَدَّثَنَا 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.