(4) باب التَّشْدِيدِ فِي الْحَلِفِ بِغَيْرِ اللَّهِ تَعَالَى
It was narrated that Ibn 'Umar said:
"The Messenger of Allah said: 'Whoever swears, let him not swear by anything other than Allah.'" The Quraish used to swear by their forefathers, and so he said: "Do not swear by your forefathers."
Reference: Sunan an-Nasa'i 3764
In-book reference: Book 35, Hadith 4
English translation: Vol. 4, Book 35, Hadith 3795