Hadith 23

Bulugh al-Maram

Book 7: Business Transactions
وَلِأَبِي دَاوُدَ: { مَنْ بَاعَ بَيْعَتَيْنِ فِي بَيْعَةٍ فَلَهُ أَوَكَسُهُمَا, أَوْ اَلرِّبَا } 1‏ .‏
Abu Dawud has:

"If anyone makes to transactions combined in one he is entitled to take the lower price, or (it is considered) usury."