Reference : Hisn al-Muslim 173

مُطِـرْنا بِفَضْـلِ اللهِ وَرَحْمَـتِه
Muṭirnā bifaḍlillāhi wa raḥmatih.

We have been given rain by the grace and mercy of Allah. 

Reference:
Al-Bukhari 1/205, Muslim 1/83.

Chapter: After rainfall