Allah Almighty, exalted be His praise, says: "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 / Q11081
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
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
(a) Yahya bin Yahya from Sufyan bin Uyaynah, (b) Imam al-Shafi'i (may Allah have mercy on him) from Sufyan bin Uyaynah, (c) Sayyiduna Abdullah bin Abbas (may Allah be pleased with him) said: When the Prophet (peace and blessings be upon him) came to Madinah, the people of Madinah used to make advance sales (salaf) in dates for two and three years. Thereupon the Messenger of Allah (peace and blessings be upon him) said: "Whoever makes an advance sale, let him do so for a known measure, a known weight and a known term."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11083
Sayyiduna Abdullah bin Abbas (may Allah be pleased with him) said: We see no harm in an advance sale (salaf) for silver in exchange for something, with the silver paid in cash.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11084
Sayyiduna Abdullah bin Umar (may Allah be pleased with him) said: An advance sale in wheat at a known price and for a known term is permissible, provided the sale does not concern an unripe crop.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11086