Sayyiduna Abdullah bin Abbas (may Allah be pleased with him) said: I bear witness that an advance sale (salaf), if it is for a fixed and known term, has been made lawful by Allah Almighty, and He has given permission for it. Then he recited the following verse: "O you who believe, when you contract a debt for a specified term, write it down" [Surah al-Baqarah: 282].
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11087