It is mustahabb to expose oneself to the rain and let some of it flow over one’s body, because of the proven report from Anas (may Allah be pleased with him) who said: When we were with the Messenger of Allah (blessings and peace of Allah be upon him) it rained. The Messenger of Allah (blessings and peace of Allah be upon him) lifted part of his garment so that the rain could fall on him. We said: O Messenger of Allah, why did you do that? He said: “Because it has just come from its Lord, may He be glorified and exalted.” Narrated by Muslim.
Ibn Sina in his Canon said, “Rain water is the best of waters, especially when it falls during summer or during a thunderstorm.”