At-Tawbah 9:43

AAafa Allahu AAanka lima athintalahum hatta yatabayyana laka allatheena sadaqoowataAAlama alkathibeen

Allāh has pardoned you, [O Muḥammad, but] why did you give them permission [to remain behind]? [You should not have] until it was evident to you who were truthful and you knew [who were] the liars.

(O Prophet,) Allah has forgiven you; why did you permit them (to stay in Madīnah) before the truthful ones could become distinct to you, and you could be sure of the liars.