ENOfficial
'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.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا ابْنُ أَبِي عُمَرَ، حَدَّثَنَا سُفْيَانُ، عَنْ عَاصِمٍ، قَالَ سَمِعْتُ أَنَسًا، يَقُولُ مَا رَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم وَجَدَ عَلَى سَرِيَّةٍ مَا وَجَدَ عَلَى السَّبْعِينَ الَّذِينَ أُصِيبُوا يَوْمَ بِئْرِ مَعُونَةَ كَانُوا يُدْعَوْنَ الْقُرَّاءَ فَمَكَثَ شَهْرًا يَدْعُو عَلَى قَتَلَتِهِمْ .
sunnah.com