وَالْمُسَافِرُ إِذَا نَسِيَ الْمَاءَ فِي رَحْلِهِ فَتَيَمَّمَ وَصَلَّى، ثُمَّ ذَكَرَ الْمَاءَ فِي الْوَقْتِ: لَمْ يُعِدْ صَلَاتَهُ عِنْدَ أَبِي حَنِيفَةَ وَمُحَمَّدٍ. وَقَالَ أَبُو يُوسُفَ: يُعِيدُ.
Meaning: If a traveller forgets water in his luggage, prays with tayammum, and remembers the water during the prayer time, Abu Hanifa and Muhammad do not require repeating the prayer; Abu Yusuf requires repetition.
The disagreement concerns how forgotten-but-present water affects legal ability to purify with water.