Aisha (ra) reported, that when the Messenger of Allah (saw) saw rain, he would say: “Allahumma sayyiban naafi‘an (O Allah, make it a beneficial downpour).”
Source: Sahih al-Bukhari
Our beloved Prophet (saw) had a divine connection with Allah (swt). There were countless miracles that occurred during his lifetime. For example there is a hadith narrated by Anas ibn Malik (ra) in Sahih al-Bukhari (Book 73 Hadith 115).
Anas Ibn Malik (ra) narrates, A man came to the Prophet (saw) on a Friday while the Prophet (saw) was delivering his sermon in Madinah. The man said, “There is a lack of rain, so please invoke Allah to bless us with the rain.” At once, the Prophet (saw) looked at the sky. There were no clouds in sight and the sky was filled with sunshine. The Prophet (saw) then invoked Allah (swt) for rain. After a little bit, clouds started gathering in the sky over Madinah. It rained so much that the valleys in Madinah started flowing with water and the rain even continued until the next Friday. Now, the same man who had asked the Prophet (saw) once again asked him except this time he asked the Prophet (saw) if he could make duah for the rain to stop. The Prophet (saw) smiled and raised his hands and asked Allah (swt) twice or thrice, “O Allah! Please let it rain round about us and upon us.” Right away, the clouds started dispersing to the right and left of Madinah. This was just one of the miraculous events in our beloved Messenger’s (saw) life.
The rain itself has very many blessings, and there are hadith concerning the benefits and virtues of it.
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 (ra) who said: When we were with the Messenger of Allah (saw) it rained. The Messenger of Allah (saw) 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.”
Source: Sahih al-Muslim (898).
The time when it rains is a miracle in itself. Rain is also a mercy from Allah (saw) to His slaves, when the means of goodness are abundant, and it is a time when it is thought that duah’s will be answered.