Assalaamu alaykum, dear Shaykh. I sent you a question (no. 2634677) earlier. Shaykh, I know that marriage between cousins is permitted and that those hadiths discouraging marriage between relatives are inauthentic. Shaykh, I saw a YouTube video of Dr. Zakir Naik regarding consanguineous marriage wherein he quoted Dr. Ahmad Sakr as saying, “Our beloved Prophet (sallallaahu 'alayhi wa sallam) said, 'Do not marry generation after generation among first cousins.'” Shaykh, I want to know whether that hadith is authentic or not. Thank you very much.
All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu 'alayhi wa sallam, is His slave and Messenger.
We have not come across any scholar who mentioned the hadeeth that you asked about in your question. However, some scholars stated that the ahaadeeth that are mentioned about the prohibition of marrying among relatives are not authentic.
For instance, the prominent scholar Ibn Al-Mulaqqin said in Al-Badr Al-Muneer, “It was reported that the Prophet, sallallaahu 'alayhi wa sallam, said, 'Do not marry a close relative because the offspring will be weak.' Al-Qaadhi Al-Husayn and the Imaam al-Haramayn (Al-Juwayni) reported it also and said, “It is inauthentic.”
As regards Ibn As-Salah, he said, “I did not find a reliable source for this hadeeth. He also commented on it saying, 'The offspring will be weak'; that is because his lust cannot be completely fulfilled with a female relative.”
Al-Qaadhi Al-Husayn mentioned another hadeeth with the same meaning, which is, 'Marry strangers, (do not marry relatives), otherwise the offspring will be weak.'
I have not seen any reliable narration in this regard in any Hadeeth book, except what I read in Ghareeb Al-Hadeeth complied by Ibraaheem al-Harbi from 'Abdullaah ibn Al-Mu'ammil from Ibn Abi Mulaykah that 'Umar, may Allaah be pleased with him, said to the tribe of As-Saa'ib, 'Your offspring has become weak; so marry stangers.'” [End of quote]
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Allaah knows best.