Check this one out, He’s Salafi:I'm not going by what that website says as I don't trust them. the guy behind it is Faraz Rabbani. he is a Sufi who promotes things like mawlid, he is unreliable.

Umar on Inbreeding: Do not to marry within bloodlines, close cousins

It’s a well known saying by Omar bin Khattab. I actually first read about it in my book of marriage by a Hanbali scholar years ago. I didn’t learn about this by Faraz. Also, Faraz also illustrates this is a Hanbali view with actual sources.
anyways do your own research, but Omar bin Khattab did indeed warn against frequent generational marriages. This is a fact.
Also, Imam Shafi said:
Al-Shafi’i said, “Whenever the people of a household do not allow their women to marry men outside of their line, there will be fools among their children.”
Source: al-Talkhīṣ al-Ḥabīr 1371
عَنْ ابْنِ أَبِي مُلَيْكَةَ قَالَ قَالَ عُمَرُ بن الخطاب رضي الله عنه لِآلِ السَّائِبِ قَدْ أَضْوَأْتُمْ فَانْكِحُوا فِي النَّوَابِغِ يَعْنِي تَزَوَّجُوا الْغَرَائِبَ
قَالَ الشَّافِعِي أَيُّمَا أَهْلِ بَيْتٍ لَمْ تَخْرُجْ نِسَاؤُهُمْ إلَى رِجَالٍ غَيْرِهِمْ كَانَ فِي أَوْلَادِهِمْ حُمْقٌ
1371 التلخيص الحبير