Ibn Jurayj (may Allah have mercy on him) said to Ata (may Allah have mercy on him): What is muhaqalah? He said: Muhaqalah in crops is just like muzabanah in date palms; it is the same: selling the standing crop for wheat. Ibn Jurayj (may Allah have mercy on him) said: I said to Ata (may Allah have mercy on him): Did Sayyiduna Jabir (may Allah be pleased with him) explain muhaqalah to you as you have told me? He said: Yes.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10641