اللَّهُمَّ صَيِّبًا نَافِعًا
Allahumma sayyiban nafi'a
O Allah, let it be a beneficial rain.
The Prophet ﷺ would say this when he saw rain. Rain is a time when du'as are answered — the Prophet ﷺ said: "Two du'as are not rejected: the du'a at the time of the adhan, and the du'a at the time of rain."
Related Du'as
اللَّهُمَّ اسْقِنَا غَيْثًا مُغِيثًا مَرِيئًا مَرِيعًا نَافِعًا غَيْرَ ضَارٍّ عَاجِلًا غَيْرَ آجِلٍ
O Allah, send us rain that is helpful, productive, wholesome, beneficial and not harmful, soon and not delayed.
Sahihاللَّهُمَّ اجْعَلْهَا رَحْمَةً وَلَا تَجْعَلْهَا عَذَابًا
O Allah, make it a mercy and do not make it a punishment.
Hasanمُطِرْنَا بِفَضْلِ اللَّهِ وَرَحْمَتِهِ
We have been given rain by the grace and mercy of Allah.
Sahihاللَّهُمَّ اغْفِرْ لِي
O Allah, forgive me. (Du'a during rainfall is accepted.)
Hasan