Shama-il Tirmidhi Chapter 029, Hadith Number 003 (190).

Anas radiyallahu anhu says, I saw Rasoolullah sallallahu alaihe wasallam eat together musk-melon and dates.

Commentary
Some ulama have translated this also as watermelon’ instead of musk-melon, the reason being, as in the first narration, here also the cold effect is intended. But it is clear that musk-melon is meant here. The reason for eating dates is also that it gives the melon a sweeter taste. It is not necessary that one’s benefit be gauged everywhere. It is possible that this is due to some other benefits. According to this humble servant the eating of watermelon with dates has its own benefit and that is to create evenness in the effect of both, so that it becomes mild. The eating of musk-melon with dates has its own benefit and that is that if the melon is insipid and tasteless, many a time, due to it being insipid sugar is mixed before it is eaten. Therefore it is not necessary to give the same reasons for both watermelon and musk-melon.