I saw the Messenger of Allah ﷺ eating fresh dates together with watermelon.
حدثنا وهب بن جرير قال حدثني أبي قال سمعت حميدا الطويل يحدث
رأيت رسول الله ﷺ يجمع بين الرطب والخربز
Its isnad is sahih. Wahb is Ibn Jareer bin Hazim, and both of them are reliable. The hadith was narrated by Abu Dawud 3/362 no. 3836, in the Book of Foods, chapter on combining two types [of food], and Tirmidhi 4/280 no. 1843, chapter on what has been said about eating watermelon with fresh dates, where he said, hasan gharib, and Ibn Majah 2/1104 no. 3325.