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It was narrated that Ibn 'Abbas said: "The Messenger of Allah (ﷺ) said: 'It does not befit us to leave bad examples. The one who takes back his gift is like the dog which goes back to its vomit.'"
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا عَمْرُو بْنُ زُرَارَةَ، قَالَ حَدَّثَنَا إِسْمَاعِيلُ، عَنْ أَيُّوبَ، عَنْ عِكْرِمَةَ، عَنِ ابْنِ عَبَّاسٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " لَيْسَ لَنَا مَثَلُ السَّوْءِ الْعَائِدُ فِي هِبَتِهِ كَالْكَلْبِ يَعُودُ فِي قَيْئِهِ " .
sunnah.com