Sunan an-Nasa'i, Volume 2, Book 12, Hadith 1080 (DarusSalam) ⓘ
It was narrated from Anas that: The Messenger of Allah said the Qunut for one month, supplicating against one of the 'Arab tribes, then he stopped doing that."
أَخْبَرَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، قَالَ أَنْبَأَنَا مُعَاذُ بْنُ هِشَامٍ، قَالَ حَدَّثَنِي أَبِي، عَنْ قَتَادَةَ، عَنْ أَنَسٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَنَتَ شَهْرًا يَدْعُو عَلَى حَىٍّ مِنْ أَحْيَاءِ الْعَرَبِ ثُمَّ تَرَكَهُ .
also referenced under: Hadith 1079 (Reference) & Book 12, Hadith 51 (In-Book)