At the time of seeking rain, one should recite the following dua
اللّٰهُمَّ اسْقِنَا غَيْثًا مُغِيثًا، مَرِيئًا مَرِيعًا، نَافِعًا غَيْرَ ضَارٍّ، عَاجِلًا غَيْرَ آجِلٍ
O Allah! grant us rain which will be a means of assistance (for us), which will be favourable (and bring a good outcome) and which will produce good growth, and will be beneficial and not harmful (for us), grant us (the rain) quickly without any delay.
عن جابر بن عبد الله، قال: أتت النبي صلى الله عليه وسلم بواكي، فقال: اللهم اسقنا غيثا مغيثا، مريئا مريعا، نافعا غير ضار، عاجلا غير آجل، قال: فأطبقت عليهم السماء (سنن أبي داود، الرقم: 1169)
Hazrat Jaabir (radhiyallahu ‘anhu) reports that on one occasion some people came crying to Rasulullah (sallallahu ‘alaihi wasallam) (and complained of shortage in rain). Rasulullah (sallallahu ‘alaihi wasallam) then made the following dua:
اللّٰهُمَّ اسْقِنَا غَيْثًا مُغِيثًا، مَرِيئًا مَرِيعًا، نَافِعًا غَيْرَ ضَارٍّ، عَاجِلًا غَيْرَ آجِلٍ
Thereafter, the entire sky became covered with clouds (and then it began to rain).