It was narrated from 'Abbad bin Tamim from his paternal uncle that:
The Prophet (sallallaahu alayhi wa sallam) went out and prayed for rain, then he prayed two rak'ahs in which he recited loudly.
[Sunan al-Nasaa’i 1522]
The prayer for rain is termed (صلاة الاستسقاء) Salaah al-'istisqaa.
The Imaam recites loudly in it.
This is the correct opinion of al-Shaafi'i, Ahmad, Abu Thaur, and Maalik. The above hadiith is evidence for their stance.
al-'ishraaf alaa madhaahib al-'ulamaa' li Ibn al-Mundhir
Volume 2 Page 190