Sahih Muslim Book 15, Hadith Number 4049.

Chapter : It is excellent to break the vow if one finds it better doing that which is against this vow one should expiate it.

Zahdam al-Jarmi reported: I visited Abu Musa and lie was eating fowl’s meat. The rest of the hadith is the same with this addition that he (the Holy Prophet) said: By Allah, I did not forget it.