Musannaf Ibn Abi Shayba 3:2633

Ibn Fudayl narrated to us, from Abd al-Rahman ibn Ishaq, from al-Nu'man ibn Sa'd, from his brother Abd Allah ibn Sa'd, who said: When Ibn Umar would swear an oath, he would say: "We are then worse than those who do not perform ghusl on the day of Jumu'ah."