The messenger of Allah, sallallahu aleyhi wa sallam was reported to have liked eating grapes and watermelon. Also, Allah has mentioned grapes in six instances in the Quran while listing the various bounties that He had bestowed in His slaves in this and the Next Life. Grapes are among the best and most beneficial fruits, and they are eaten fresh, dried, green and ripe. Further, Grapes are a type of fruit and nutritious food, along with their benefits as a remedy and a drink. Wetness and hotness are among the characteristics of grapes just as other grains. The best type of grapes are the big grapes that are full of juice, while white grapes are better than the black when both are equal in sweetness. Also, eating fresh grapes two or three days after harvest is better than eating them immediately otherwise they cause diarrhea and flatulence. The grapes are left on their hanging grapevines until their skins becomes thin. They are a source of good nutrition, just like rains and figs, and also strengthen the body.
When the seeds of the grapes are removed, the grapes become a more effective laxative. Eating grapes excessively causes headache, unless one also eats sour pomegranate. Grapes in general are a good laxative and provide a good source of nutrition. The best types of fruits are three: grapes, dates and figs.
Taken from: "Healing with the Medicine of the Prophet," by Ibn ul-Qayyim