Musnad Ahmad 1153:1552

'Abd al-Rahman narrated to us, from Sufyan, from al-Miqdam ibn Shurayh, from his father, from 'Aisha, that the Messenger of Allah (peace and blessings of Allah be upon him), when he saw a formation [of clouds] from one of the horizons of the sky, would leave his work, even if he were in his prayer. Then he would say, "O Allah, I seek refuge in You from the evil of what is in it." If Allah removed it, he would praise Allah. And if it rained, he would say, "O Allah, a beneficial downpour."