Musnad Ahmad 27:226

Yahya narrated to us, from Ubaydullah, and Muhammad bin Ubayd, who said: Ubaydullah narrated to us, from Nafi’, from Ibn Umar, from the Prophet, peace and blessings of Allah be upon him: "When a slave performs the worship of his Lord, Blessed and Exalted is He, well, and is sincere to his master, he shall have his reward twice."