Khalid bin Umayr narrated: We went on a campaign with Utbah bin Ghazwan (may Allah be pleased with him) and conquered al-Ubullah, and there we saw walnut shells lying about. We said: We have never seen stones which we could break open and eat (meaning we had never come across walnuts before).
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14349