Musnad Ahmad 1153:1361

Muhammad ibn Ja‘far narrated to us, he said: Sa‘id and ‘Abd al-Wahhab narrated to us, from Sa‘id, from Abu Ma‘shar, from Ibrahim, from al-Aswad, from ‘A’ishah, she said: "When the Messenger of Allah, peace and blessings of Allah be upon him, wanted to perform ghusl (ritual bath), he would begin with his palms and wash them. Then he would pour with his right hand onto his left, and wash his private parts. When he had cleansed them, he would strike his hand against the wall, then wash it. Then he would face the water for ablution and pour the water over himself."