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

Ibn Abi Mulaykah reported: Umar ibn al-Khattab, may Allah be pleased with him, said to the house of Al-Sa’ib, “You have become frail, so marry intelligent people unrelated to you.”

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