Musnad Ahmad 1153:2149

Mus'ab ibn al-Miqdam narrated to us, he said: Isra'il narrated to us, from al-Miqdam, from his father, that he asked Aisha: "What did the Messenger of Allah, may Allah bless him and grant him peace, do before he would go out?" She said: "He would pray the two rak'ahs before the dawn, then he would go out to the prayer. And when he entered, he would use the siwak."