Abu al-Bakhtari says: I asked Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) about "salam fi al-nakhl" (a salam sale of dates still on the palm). He said that the Messenger of Allah (peace and blessings be upon him) "forbade selling dates until they become fit to eat or until they are weighed." I asked: What is weighing it? A man among the people said: It means estimating.
(b) In the narration of Adam: that man said: With what is it weighed? Then the man sitting beside him said: It means estimating.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10604