At the time of seeking rain, one should recite the following dua
اللَّهُمَّ أَغِثْنَا، اللَّهُمَّ أَغِثْنَا، اللَّهُمَّ أَغِثْنَا
O Allah! Bless us with rain! O Allah! Bless us with rain! O Allah! Bless us with rain!
عن أنس بن مالك: أن رجلا، دخل المسجد يوم جمعة من باب كان نحو دار القضاء، ورسول الله صلى الله عليه وسلم قائم يخطب، فاستقبل رسول الله صلى الله عليه وسلم قائما، ثم قال: يا رسول الله، هلكت الأموال وانقطعت السبل، فادع الله يغيثنا، فرفع رسول الله صلى الله عليه وسلم يديه، ثم قال: اللهم أغثنا، اللهم أغثنا، اللهم أغثنا قال أنس: ولا والله، ما نرى في السماء من سحاب، ولا قزعة وما بيننا وبين سلع من بيت ولا دار، قال: فطلعت من ورائه سحابة مثل الترس فلما توسطت السماء انتشرت، ثم أمطرت (صحيح البخاري، الرقم: 1014)
Hazrat Anas (radhiyallahu ‘anhu) reports that once a person entered the musjid on the day of Jumuah from the door facing daarul qadha (the house sold to pay off the debt of Hazrat Umar (radhiyallahu ‘anhu)). He entered while Rasulullah (sallallahu ‘alaihi wasallam) was delivering the Khutbah. The man stood in front of Rasulullah (sallallahu ‘alaihi wasallam) and said, “O Rasulullah (sallallahu ‘alaihi wasallam)! The livestock are dying and the roads are cut off (due to the animals being too weak for travel); so make dua to Allah Ta‘ala for rain.” Rasulullah (sallallahu ‘alaihi wasallam) raised both his hands and made dua saying:
اللَّهُمَّ أَغِثْنَا، اللَّهُمَّ أَغِثْنَا، اللَّهُمَّ أَغِثْنَا
Hazrat Anas (radhiyallahu ‘anhu) further says, “By Allah, we could not see any cloud in the sky nor any sign of rain, and there was no house or dwelling between us and (the mountains of) Sala’. Then a cloud (round in shape) like a shield appeared from behind it (i.e. the Sala’ Mountain). When it came in the middle of the sky, it spread out (and grew until it covered the entire sky) and then it began to rain.”