It was narrated from Salim, from his father, that on one occasion the Prophet heard 'Umar saying:
"By my father and by my mother." He said: "Allah forbids you to swear by your forefathers." 'Umar said: "By Allah, I never swore by them again, whether saying it for myself or reporting it of others."
Reference: Sunan an-Nasa'i 3766 In-book : Book 35, Hadith 6 English translation : Vol. 4, Book 35, Hadith 3797
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