I said, "Messenger of Allah, my cousin comes to me, and I swear that I will not give him anything nor maintain ties with him."He replied, "Make expiation for your oath."
حدثنا محمد بن أبي عمر العدني حدثنا سفيان بن عيينة حدثنا أبو الزعراء عمرو بن عمرو
قلت يا رسول الله يأتيني ابن عمي فأحلف أن لا أعطيه ولا أصله قال كفر عن يمينك
Its chain of transmission is sahih. Nasai narrated it in the chapter on oaths and vows, section on expiation after breaking an oath, hadith no. 3819, from the hadith of Sufyan, through him. It is also cited in Musnad Humaydi, hadith no. 885, in my edition.