Musnad Abu Ya'la al-Mawsili 42:181

Abu Khaythama narrated to us, Abd al-Rahman ibn Mahdi narrated to us, Shu'bah narrated to us, from Anas ibn Sirin, from his brother Ma'bad ibn Sirin, from Abu Sa'id al-Khudri, from the Prophet, peace and blessings of Allah be upon him, who said: "It is of no consequence to you if you do not do it, for it is only the decree."