Sunan an-Nasa'i 4549

It was narrated that Ibn 'umar said:"The Messenger of Allah forbade Muzabanah, which refers to when a man sells the dates of his grove while they are still on the trees, for a measure of dry on the tree, for a measure of dry dates, estimating the amount( of dates on the trees). Or, if it is grapes, he sells them when they are still on the vines, for a measure of raisins, estimating the amount (of grapes on the vines). Or if it is grain in the field, he sells it for grain that has been harvested, estimating the amount (of grain in the fields). He forbade all of that