Ibn Abbas (Allah be pleased with them) reported Allah's Messenger (ﷺ) as saying:

He who buys foodgrain should not sell it, until he has weighed it (and then taken possession of it). I (Tawus) said to Ibn Abbas (Allah be pleased with them): Why is it so? Thereupon he said: Don't you see that they (the people) sell foodgrains against gold for the stipulated time. Abu Kuraib did not make any mention of the stipulated time.

حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، وَأَبُو كُرَيْبٍ وَإِسْحَاقُ بْنُ إِبْرَاهِيمَ قَالَ إِسْحَاقُ أَخْبَرَنَا وَقَالَ الآخَرَانِ، حَدَّثَنَا وَكِيعٌ، عَنْ سُفْيَانَ، عَنِ ابْنِ طَاوُسٍ، عَنْ أَبِيهِ، عَنِ ابْنِ عَبَّاسٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏

‏ مَنِ ابْتَاعَ طَعَامًا فَلاَ يَبِعْهُ حَتَّى يَكْتَالَهُ ‏

‏ ‏.‏ فَقُلْتُ لاِبْنِ عَبَّاسٍ لِمَ فَقَالَ أَلاَ تَرَاهُمْ يَتَبَايَعُونَ بِالذَّهَبِ وَالطَّعَامُ مُرْجَأٌ وَلَمْ يَقُلْ أَبُو كُرَيْبٍ مُرْجَأٌ ‏.‏