Ikrimah (may Allah have mercy on him) says about Ibn Abbas (may Allah be pleased with them both) that he regarded as disliked a salam sale made until the harvesting of the crop, or until it has fully grown, or until the crop is on the threshing floor, and he would say: "Fix the term by naming a month."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11116