(36) باب اشْتِرَاءِ التَّمْرِ بِالرُّطَبِ
it was narrated that Sa'd said:
"The Messenger of Allah was asked about (buying) fresh dates with dried dates, and he said to those who were around him: 'Will fresh dates decrease (in weight or volume) when they dry out?" they said Yes,' so he forbade that."
Reference: Sunan an-Nasa'i 4545
In-book reference: Book 44, Hadith 97
English translation: Vol. 5, Book 44, Hadith 4549