Musnad Ahmad 1153:972

'Affan narrated to us, Shu'bah narrated to us, Ash'ath ibn Sulaym said: He informed me that he heard his father narrating from Masruq, from 'Aishah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to love starting with the right in all his affairs: in his purification, in combing his hair, and in his footwear. He said: Then I asked him in Kufa, and he said: "Starting with the right, as much as he is able."