Musnad Ahmad 30:1967

Aswad narrated to us, Sharik narrated to us, from Suhayl, from his father, from Abu Hurayrah, who attributed it to the Prophet (ﷺ): "He forbade *al-muhaqalah*: and it is the purchasing of crops while they are in their ears with wheat. And he forbade *al-muzabanah*: and it is the purchasing of fruits with dates."