He told me, whom I trust, he narrated from Sufyan, and he said just as I have said, and regarding the period he said: "That the period should be known."
Hadith Referenceمسند الإمام الشافعي / كتاب البيوع / 1439
Hadith Takhrijاخرجه البخارى البيوع، باب السلم في وزن معلوم (2240) ومسلم ، المساقاة، باب السلم (1604)۔