Sayyiduna Abdullah bin Abbas (may Allah be pleased with him) said: I bear witness that the advance sale (salaf) whose specified term is guaranteed has been made lawful by Allah Almighty, and He has permitted it. Then he recited this 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 / 11081