Bulugh al-Maram
Book 7, Hadith 23
Arabic hadith text
وَلِأَبِي دَاوُدَ: { مَنْ بَاعَ بَيْعَتَيْنِ فِي بَيْعَةٍ فَلَهُ أَوَكَسُهُمَا, أَوْ اَلرِّبَا } 1 .1 - حسن. رواه أبو داود ( 3460 ).
Translation
Abu Dawud has
"If anyone makes to transactions combined in one he is entitled to take the lower price, or (it is considered) usury."