Sahih Muslim | Book 5 | Chapter 20

Narrated by Abu Mas'ud
Abu Mas'ud reported: We were commanded to give charity (despite the fact.) that we were coolies. Abu 'Aqil donated half a sa'. And there came another man with more than this. The hypocrites said: Verily Allah does not stand in need of the charity of this, and the second one has done nothing but only made a show (of his charity). Then this verse was revealed. "Those who scoff at the voluntary givers of charity among the believers as well as those who cannot find anything (to give) but with their hard labour"(ix. 80). And Bishr did not utter the word Muttawwi'in.

Narrated by Shu'ba
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.