When it rains falling in the earth

When it rains, God is making it as a favor and mercy for humans, humans aren't exception. Even the mention at  Al-Qur'an.

Have those who disbelieved not considered that the heavens and the earth were a joined entity, and We separated them and made from water every living thing? Then will they not believe? (QS. Al-Anbiya': 30) 

However, God also made ​​the rain and an abundance of water as punishment for dissent, as it happened to the people of Nabi Nuh 'Alaihissalam.

And [mention] Noah, when he called [to Allah ] before [that time], so We responded to him and saved him and his family from the great flood.(QS. Al-Anbiya': 76) 

And We saved him from the people who denied Our signs. Indeed, they were a people of evil, so We drowned them, all together. (QS. Al-Anbiya': 77) 


So when it rains, the Muslims who hope that rain brings good and be a blessing as was taught Rasulullah Shallallahu 'Alaihi Wasallam. Prayer / Dzikir are: 

اللَّهُمَّ صَيِّبًا نَافِعًا

ALLAHUMMA SHAYYIBAN NAAFI'A

Ya Allah, (make this rain)  rain that brings the benefits (good).

Narrated from 'Aisyah Radhiyallahu 'Anha, 

 أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ كَانَ إذَا رَأَى الْمَطَرَ قَالَ : اللَّهُمَّ صَيِّبًا نَافِعًا

"Is Nabi Shallallahu 'Alaihi Wasallam when he saw the rain praying:  ALLAHUMMA SHAYYIBAN NAAFI'A Ya Allah, (make this rain)  rain that brings the benefits (good)." (HR. Al-Buhari)