Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) said that the Prophet (peace and blessings be upon him) "forbade selling grain until the sa' has been applied to it twice"; that is, it is to be measured twice, the buyer's sa' and the seller's sa': both should measure before it is sold.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10700