Musnad Ahmad 1153:577

Ali ibn Bahr narrated to us, he said: Isa ibn Yunus narrated to us, al-Awza'i narrated to us, from al-Zuhri, from al-Qasim ibn Muhammad, from Aisha, that the Prophet, peace and blessings of Allah be upon him, used to say when he saw the rain: "O Allah, make it a beneficial and wholesome downpour."