وَإِذَا وُجِدَ فِي الْبِئْرِ فَأْرَةٌ مَيِّتَةٌ أَوْ غَيْرُهَا، وَلَا يَدْرُونَ مَتَى وَقَعَتْ، وَلَمْ تَنْتَفِخْ وَلَمْ تَتَفَسَّخْ: أَعَادُوا صَلَاةَ يَوْمٍ وَلَيْلَةٍ إِذَا كَانُوا تَوَضَّؤُوا مِنْهَا، وَغَسَلُوا كُلَّ شَيْءٍ أَصَابَهُ مَاؤُهَا
Translation: If a dead mouse or another animal is found in a well and nobody knows when it fell in, and it has neither swollen nor decomposed, those who made wudu from it repeat one day and night of prayers and wash things touched by its water.
The question extends beyond the well’s current impurity to past prayers performed using its water. The one-day-and-night estimate is the view of Abu Hanifa in the disputed case; the two companions’ view follows explicitly.