Abu Hassan narrates that Sayyiduna Ibn Abbas (may Allah be pleased with them both) was asked about salaf (a sale with advance payment), and he said: "I bear witness that Allah has made it lawful, and He revealed concerning it the longest verse in the Book of Allah: 'O you who believe, when you contract a debt for a specified term, write it down' [Surah al-Baqarah: 282]."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12847