Short answer
For men, letting trousers, a thobe or a lower garment hang below the ankles out of pride is haram. Without pride, classical Hanafi texts call it makruh, and many Deobandi muftis say pride is hard to rule out and advise avoiding it altogether. The rule is for men only: women are told to cover their ankles.
Isbal (letting the garment fall below the ankles) is haram for men when done from pride. Without pride, al-Fatawa al-Hindiyya calls it makruh tanzihi, while many later Hanafi muftis treat it as makruh tahrimi today. Prayer in such clothes is valid.
The four schools agree on this.
Explanation
The Prophet ﷺ said that whatever of the lower garment is below the ankles is in the Fire, and that Allah will not look on the Day of Judgement at a man who drags his garment out of pride. When Abu Bakr said his garment kept slipping down unless he held it, the Prophet told him: you are not one of those who do it out of pride.
From these hadiths the four schools agree on the basic picture: dragging one's clothes from pride is haram, and without pride it is disliked. Some scholars, including some Hanbalis, held that it is haram even without pride. The Hanafi muftis of our time note that long hems are usually a matter of fashion, which is close to showing off, so they advise men to keep all trousers, jeans and thobes above the ankle bone.
In prayer. The prayer is valid either way. The Hanafi school dislikes tucking or folding clothes during prayer (kaff al-thawb), and muftis differ on whether folding up a long hem counts. The cleanest solutions are trousers of the right length or folding at the waistband before you pray. See how to pray.
For women. The ruling is reversed. A woman's feet are covered, so her clothes may be long. Women's dress is covered in the women section and on the hijab page.
The evidence
Quran, Surah Luqman 31:18
وَلَا تُصَعِّرْ خَدَّكَ لِلنَّاسِ وَلَا تَمْشِ فِى ٱلْأَرْضِ مَرَحًا ۖ إِنَّ ٱللَّهَ لَا يُحِبُّ كُلَّ مُخْتَالٍۢ فَخُورٍۢ ﴿١٨﴾
And do not turn your cheek [in contempt] toward people and do not walk through the earth exultantly. Indeed, Allah does not like everyone self-deluded and boastful.
Pride in dress and walk is disliked by Allah.
Sahih al-Bukhari 5787
Sahih... ما أسفل من الكعبين من الإزار ففي النار ...
"Whatever of the lower garment is below the ankles is in the Fire."
The main warning about isbal.
Sahih al-Bukhari 3665
Sahih... إنك لست تصنع ذلك خيلاء ...
"You are not one of those who do it out of pride."
Shows that pride is what makes it haram.
Sahih Muslim 106
Sahih... ثلاثة لا يكلمهم الله يوم القيامة المنان الذي لا يعطي شيئا إلا منه ...
"Three people Allah will not speak to on the Day of Judgement: the one who reminds people of his gifts, the one who sells with false oaths, and the one who lets his lower garment hang down."
Lists the musbil among those warned.
Where the ruling is recorded
- al-Fatawa al-Hindiyya, Kitab al-Karahiyya, chapter on clothing. Isbal without pride is makruh tanzihi.
- al-Durr al-Mukhtar with Radd al-Muhtar, Kitab al-Salah, chapter on what invalidates the prayer and what is disliked in it. Tucking up clothes in prayer (kaff al-thawb).
- al-Mughni (Ibn Qudama), Kitab al-Salah, section on clothing in prayer. The Hanbali discussion of isbal with and without pride.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.