(65) Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh))
كتاب التفسير
(4) Allah's Saying: "And say ... Hittatun..." [i.e., (O Allah) forgive our sins] (V.7:161)
(4) باب قَوْلِهِ ‏{‏وَقُولُوا حِطَّةٌ‏}‏
Sahih al-Bukhari 4641 

Narrated Abu Huraira:

Allah's Messenger (ﷺ) said, "It was said to the children of Israel, 'Enter the gate in prostration and say Hitatun. (7.161) We shall forgive you, your faults.' But they changed (Allah's Order) and entered, dragging themselves on their buttocks and said, 'Habatun (a grain) in a Sha'ratin (hair).

حَدَّثَنَا إِسْحَاقُ، أَخْبَرَنَا عَبْدُ الرَّزَّاقِ، أَخْبَرَنَا مَعْمَرٌ، عَنْ هَمَّامِ بْنِ مُنَبِّهٍ، أَنَّهُ سَمِعَ أَبَا هُرَيْرَةَ ـ رضى الله عنه ـ يَقُولُ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ قِيلَ لِبَنِي إِسْرَائِيلَ ‏{‏ادْخُلُوا الْبَابَ سُجَّدًا وَقُولُوا حِطَّةٌ نَغْفِرْ لَكُمْ خَطَايَاكُمْ‏}‏ فَبَدَّلُوا فَدَخَلُوا يَزْحَفُونَ عَلَى أَسْتَاهِهِمْ وَقَالُوا حَبَّةٌ فِي شَعَرَةٍ ‏"‏‏.‏
Reference : Sahih al-Bukhari 4641
In-book reference : Book 65, Hadith 163
USC-MSA web (English) reference : Vol. 6, Book 60, Hadith 165
  (deprecated numbering scheme)