Musnad Ahmad 1153:956

Ahmad ibn al-Hajjaj narrated to us, he said: Abdullah narrated to us, he said: Ubaydullah ibn Umar informed us, from Nafi’, from al-Qasim ibn Muhammad, from Aisha, that the Messenger of Allah, peace and blessings of Allah be upon him, when he saw the rain, would say: "O Allah, a beneficial downpour."