From Sahih Muslim (Hadith 1550):
Asim reported - I heard Anas saying:Never did I ace the Messenger of Allah (ﷺ) so much grieved (at the loss of a) small army as I saw him grieved at those seventy men who were called" reciters" (and were killed) at Bi'r Ma'una; and he invoked curse for full one month upon their murderers
Reference: Muslim 1550