Sayyiduna Abdullah bin Abbas (may Allah be pleased with him) said concerning the verse of the Qur'an, "O you who believe, when you contract a debt for a specified term" [Surah al-Baqarah: 282]: It refers to a transaction in wheat by a known measure.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11082