Musnad Ahmad 1153:1527

'Abd al-Rahman ibn Mahdi narrated to us, from Shu'bah, from Ash'ath ibn Abi al-Sha'tha', from his father, from Masruq, from 'Aishah, who said: The Prophet, peace and blessings of Allah be upon him, used to like starting with the right in all his affairs: in his combing, in his purification, and in his footwear. Shu'bah said: Then I asked him after that, and he said: The Prophet, peace and blessings of Allah be upon him, used to love—or it used to please him—to start with the right as much as he was able.