Musnad Ahmad 1153:576

Yazid ibn Abd Rabbih narrated to us, he said: Al-Walid ibn Muslim narrated to us, from Al-Awza'i, from Nafi', from Al-Qasim ibn Muhammad, from Aisha, that the Messenger of Allah, 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."