once
اللَّهُمَّ صَيِّباً نَافِعاً
Edition transliteration
Allahumma sayyiban nafiAAa.
Meaning
O Allah, may it be a beneficial rain cloud.
Source and references
Hisn al-Muslim, entry 172.The reference is copied from the edition; separate editorial verification is not complete yet.
Source edition: Fortress of the Muslim. Open source ↗