صحيح مسلم

Abd Sa'id reported: Bilal (Allah be pleased with him) came with fine quality of dates. Allah's Messenger (ﷺ) said to him: From where (you have brought them)? Bilal said: We had inferior quality of dates and I exchanged two sa's (of inferior quality) with one sa (of fine quality) as food for Allah's Messenger (ﷺ), whereupon Allah's Messenger (ﷺ) said: Woe! it is in fact usury; therefore, don't do that. But when you intend to buy dates (of superior quality), sell (the inferior quality) in a separate bargain and then buy (the superior quality). And in the hadith transmitted by Ibn Sahl there is no mention of" whereupon".