Firstly: The Meaning of Seeking Rain
Literally, istisqa’ means seeking watering, as it is the Form X verbal noun of the root saqy, which means to provide water for drinking and irrigation.
Legally, istisqa’ is asking Allah the Exalted in a particular manner to send rain when there is a need.
Secondly: The Ruling of Praying for Rain
The prayer for rain is sunnah. This is the position of the majority: the Malikis, Shafi`is, and Hanbalis. It is also the position of Muhammad and Abu Yusuf of the Hanafi school. The majority of scholars of the Salaf and Khalaf hold that it is legislated.
Thirdly: The Ruling of Repeating the Prayer for Rain
If people prayer for rain and it does not fall, then repeating the prayer is legislated by agreement of the four schools of jurisprudence.
Fourthly: If it Rains Before Gathering to Pray for Rain
Scholars have differed regarding whether the prayer for rain should be offered if it rains before people gather to pray for it. They are of two positions:
The first position: If it rains before they gather to pray for it, it is mustahabb that they still gather and pray. This is the position of the majority: the Hanafis, Shafi`is, and Hanbalis  However, they hold that if they had not prepared themselves to head out, then they do not pray. If they had prepared themselves to head out, then they head out and pray. .
The second position: If it rains before they gather, then they do not gather to pray. This is one position among Shafi`is, and it was declared sound by Ibn al-Salah. It is the position of choice of Ibn Qudamah and Ibn `Uthaymin.