The Book of Zakat
وَحَدَّثَنَا مُحَمَّدُ بْنُ بَشَّارٍ، حَدَّثَنِي سَعِيدُ بْنُ الرَّبِيعِ، ح وَحَدَّثَنِيهِ إِسْحَاقُ بْنُ مَنْصُورٍ، أَخْبَرَنَا أَبُو دَاوُدَ، كِلاَهُمَا عَنْ شُعْبَةَ، بِهَذَا الإِسْنَادِ . وَفِي حَدِيثِ سَعِيدِ بْنِ الرَّبِيعِ قَالَ كُنَّا نُحَامِلُ عَلَى ظُهُورِنَا .This hadith has been narrated by Shu'ba with the same chain of transmitters and in the hadith transmitted by Sa'id b. al Rabi (the words are):" We used to carry loads on our backs
From Sahih Muslim, whose reports the scholars accept as authentic.