Musnad Ahmad 28:549

Ya'qub and Sa'd narrated to us, they said: My father narrated to us, from Ibn Ishaq—meaning Muhammad—he said: Abd al-Rahman ibn al-Harith narrated to me, from Amr ibn Shu'ayb, from his father, from his grandfather, who said: The Messenger of Allah, peace and blessings of Allah be upon him, decreed, saying: "There is no Shighar in Islam."