Musnad Ahmad 1153:135

'Abdah narrated to us, Mis'ar narrated to us, from al-Miqdam ibn Shurayh, from his father, from 'Aishah, that the Prophet, may Allah bless him and grant him peace, when he saw rain, would say: "O Allah, a beneficial downpour." He said: And I asked 'Aishah with what thing the Prophet, may Allah bless him and grant him peace, would begin when he entered his house. She said: "With the siwak."