Its narrated by Imaam Muslim in his authentic compilation from the hadith of Anas, He said: It rained upon us as we were with the Messenger of Allah (may peace be upon him). The Messenger of Allah (may peace be upon him) unveiled his garment (from a part of his body) until the rain fell on him. We said: Messenger of Allah, why did you do this? He said: Because it (the rainfall) has just come from the Exalted Lord.
It is a common sight to see people frantically running to find cover at the first sight of rain. Allah has complete control over everything. In an instant good can turn into bad and bad can turn into good. Rain is a blessing from Allah that many parts of the world wish to receive and yet are living in dry lands.
As Allah says in the Quran: Say (O Muhammad (pbuh)): "Tell me! If (all) your water were to be sunk away, who then can supply you with flowing (spring) water?" [Surah 67 Al-Mulk, Verse 30]
As we have read in the hadith above, exposing a part of your body to rain is a practice of the Prophet (peace be upon him). He rejoiced at the sight of rain as it was a mercy created by Allah and therefore we use it as a blessing. As revealed in the Quran “And we have sent down from the heavens water that is blessed” [Surah 50, Verse 9].
It is a sunnah to expose some part of your body to the rain no matter how small like your arms or your head. While following this sunnah, ones Awrah (intimate parts of your body) should not be exposed.
Let us all receive the blessing of rain with open hands and use the water wisely for our needs.
"He who holds on firmly to my Sunnah at a time when my Ummah are (grovelling) in corruption will receive a reward of a hundred martyrs."
Abu Hurayrah rahimahullah reported that Allah’s messenger sallalaahu ‘alayhi was sallam said: “When a human being dies, all of his deeds are terminated except for three types: an ongoing sadaqah, a knowledge(of Islaam) from which others benefit, and a righteous child who makes du’aa for him.” (Muslim and others)