كتاب الزكاة
12  
The Book of Zakat
(21)
Chapter: Carrying goods for payment and giving charity out of one's wages, and the stern prohibition of belittling the one who gives something small in charity
(21)
باب الْحَمْلِ بِأُجْرَةٍ يُتَصَدَّقُ بِهَا وَالنَّهْىِ الشَّدِيدِ عَنْ تَنْقِيصِ الْمُتَصَدِّقِ بِقَلِيل
Sahih Muslim 1018 b

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."
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 ‏.‏
Reference : Sahih Muslim 1018b
In-book reference : Book 12, Hadith 93
USC-MSA web (English) reference : Book 5, Hadith 2224
  (deprecated numbering scheme)