Oaths and Vows (Kitab Al-Aiman Wa Al-Nudhur)
Dawud :: Book 21 : Hadith 3267
Narrated Abdullah ibn Amr ibn al-'As:
The Apostle of Allah (peace_be_upon_him) said: A vow is binding in those things by which the pleasure of Allah is sought, and an oath to break ties of relationship is not binding.