Short answer
The Prophet ﷺ wore the ordinary dress of his people: a long shirt (qamis), which he loved most, a waist wrap (izar) with a shoulder cloth (rida), a cloak (jubba), striped Yemeni cloth (hibarah), a turban or cap, and sandals. Wearing a qamis or thobe to follow him is a rewarded sunnah of habit, not an obligation.
Following the Prophet ﷺ in dress is a sunnah of habit: rewarded with the right intention but not required. The qamis (long shirt), which matches today's thobe or long kurta, was his favourite garment.
The four schools agree on this.
Explanation
Umm Salamah said the garment the Prophet ﷺ loved most was the qamis, a long shirt reaching below the knees. Anas said the garment he most liked to wear was the hibarah, a striped cloth from Yemen. He also wore the izar (a wrap for the lower body) with a rida (a cloth over the shoulders), a Syrian jubba with narrow sleeves on a journey, a black turban, caps and leather sandals.
Colours and fabrics. He wore white and praised it, and also wore green cloaks, a black woollen cloak and red-striped cloth. His clothes were mostly cotton and wool, simple and clean. He gave up gold and silk and forbade them to men.
Is the thobe sunnah? The thobe, kandura, jubba and long kurta are today's forms of the qamis, so wearing one out of love for the Prophet earns reward. It is a sunnah of habit, so a man in trousers and a shirt that meet the rules is not sinning. What is required is covering the awrah, keeping clothes above the ankles, and avoiding silk, gold and women's styles; see the dress code for men.
His manners in dress. He started with the right side when dressing, praised Allah for new clothes and kept his dress humble. See the Sunnah way of getting dressed, the turban and sandals.
The evidence
Quran, Surah Al-Ahzaab 33:21
لَّقَدْ كَانَ لَكُمْ فِى رَسُولِ ٱللَّهِ أُسْوَةٌ حَسَنَةٌۭ لِّمَن كَانَ يَرْجُوا۟ ٱللَّهَ وَٱلْيَوْمَ ٱلْءَاخِرَ وَذَكَرَ ٱللَّهَ كَثِيرًۭا ﴿٢١﴾
There has certainly been for you in the Messenger of Allah an excellent pattern for anyone whose hope is in Allah and the Last Day and [who] remembers Allah often.
His way, including his dress, is the best example.
Jami' at-Tirmidhi 1764
Grading to be checked... كان أحب الثياب إلى رسول الله صلى الله عليه وسلم القميص ...
"The garment most beloved to the Messenger of Allah was the qamis."
The long shirt was his favourite garment.
Sahih al-Bukhari 5813
Sahih... كان أحب الثياب إلى النبي صلى الله عليه وسلم أن يلبسها الحبرة ...
"The garment the Prophet most liked to wear was the hibarah."
He also loved striped Yemeni cloth.
Sahih al-Bukhari 363
Sahih... فقضى حاجته وعليه جبة شأمية ...
"He relieved himself while he was wearing a Syrian cloak."
He wore a jubba on his journeys.
Sunan Abi Dawud 4206
Grading to be checked... وعليه بردان أخضران ...
"He was wearing two green garments."
He wore colours other than white.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Taharah, sunnahs of wudu. The Prophet's dress belongs to the sunnah of habit, which is rewarded but not required.
- al-Fatawa al-Hindiyya, Kitab al-Karahiyya, chapter on clothing. On the kinds of clothing that are recommended, permitted and disliked.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. 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.