Translation: "Yahyā ibn Ma'īn was asked if Umar ibn Sa'd was trustworthy (thiqāh), to which he replied, 'How can he be trustworthy when he killed Al-Husayn?'"

Let us take a look at Tahtheeb Al Tahtheeb (Thanks to Brother Yusef for providing this) . “Al Aijli said (regarding Umar Ibn Sa’d): He narrated from his father, and he killed Al Hussain, and he is a Tabi’i Thiqah'”

In Tarikh Dimashq: “Abu Al Barakat Al Anmati told us .. all the way to Salih Ibn Ahmed from his father: Umar Ibn Sa’d used to narrate from his father, and people would narrate from him and he is the one who killed Al Hussain” It was also said “Umar Ibn Sa’d, Tabi’i Thiqah, and he is the one who killed Al Hussain”