It was narrated from Sa'eed bin Jubair that Ibn 'Abbas said:
"For the one who makes unlawful is the swearing." (Sahih)And Ibn 'Abbas used to say: "You had the best example in the Messenger of Allah." (33:21)
English reference: Vol. 3, Book 10, Hadith 2073
Arabic reference: Book 10, Hadith 2151