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) (RU)
English reference (RU): Vol. 3, Book 10, Hadith 2073 (RU)
Arabic reference (RU): Book 10, Hadith 2151 (RU)