The Prayer for Seeking Rain (Salat Al-Istisqa)
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 80
وَ رُوِيَ: «أَنَّ اَلرَّعْدَ صَوْتُ مَلَكٍ أَكْبَرَ مِنَ اَلذُّبَابِ وَ أَصْغَرَ مِنَ اَلزُّنْبُورِ فَيَنْبَغِي لِمَنْ سَمِعَ صَوْتَ اَلرَّعْدِ أَنْ يَقُولَ سُبْحَانَ مَنْ «يُسَبِّحُ اَلرَّعْدُ بِحَمْدِهِ وَ اَلْمَلاٰئِكَةُ مِنْ خِيفَتِهِ» »
It is narrated: "The thunder is the sound of an angel that is larger than a fly and smaller than a wasp. When one hears the sound of thunder, it is recommended to say, 'Glory be to the One whom the thunder praises with His praise and (so do) the angels out of fear of Him.'"
- Ṣaḥīḥ al-Faqīh