Hafsa reported: Never did I see the Messenger of Allah (ﷺ) observing supererogatory prayer sitting till one year before his death when he would observe Nafl prayer in a sitting position, and he would recite the Surah (of the Qur'an) in such a slow-measured tone (that duration of its recital) became more lengthy than the one longer than this.
Hadith Referenceصحيح مسلم / كتاب صلاة المسافرين وقصرها / 733
´Hafsah, the wife of the Prophet (S), narrated:` "I did not see Allah's Messenger (S) praying voluntary prayers sitting until the year before he died. Then he would perform Salat for the voluntary prayers sitting, and he would recite a Surah and prolong it such that it would be longer than the longest of them."
Shaykh Dr. Abdur Rahman Freywai
1: However, by performing voluntary (nafl) prayers while sitting, your (sallallahu alayhi wa sallam) reward is not reduced to half like that of your followers; this is among your special characteristics.
Source: Sunan al-Tirmidhi – Majlis ‘Ilmi Dar al-Da‘wah, New Delhi Edition, Page: 373
Umm al-Mu'minin Sayyida Hafsa (may Allah be pleased with her) said: I never saw the Messenger of Allah (peace and blessings be upon him) performing voluntary prayers while sitting, until one year before his passing, he began to perform voluntary prayers while sitting. He would recite the surah with measured recitation (tarteel), so much so that due to his tarteel, that surah would become longer than a surah that was actually longer than it.
Hafiz Zubair Ali Zai
Hadith Authentication (Takhrij al-Hadith): [الموطأ رواية يحييٰ بن يحييٰ 137/1، ح 307، ك 8، ب 7، ح 21، التمهيد 220/6، الاستذكار : 277 ● أخرجه مسلم فواد عبدالباقی ترقیم 733، دارالسلام ترقیم 1712، من حديث مالك به] Jurisprudential Understanding (Tafaqquh): ➊ Hafiz Ibn Abd al-Barr said: «وَفِياللُّغَةِأَنَّالصَّلَاةَأَصْلُهَاالدُّعَاءُلَكِنَّالْأَسْمَاءَالشَّرْعِيَّةَأَوْلَىلِأَنَّهَاقَاضِيَةٌعَلَىاللُّغَوِيَّةِ» In language, the original meaning of salah (prayer) is supplication (du'a), but the legal (shar'i) names are preferable, because they take precedence over the linguistic names. [التمهيد 221/6] ➋ It is preferable to perform voluntary (nafl) prayers standing, and deliberately performing voluntary prayers sitting without any legal excuse is not appropriate. ➌ The recitation of the Noble Qur'an should be done slowly, in a good manner, and with reflection and contemplation. ➍ The standing (qiyam) in voluntary prayers should be as long as possible. ➎ It is recommended to recite much of the Noble Qur'an in voluntary prayers. ➏ If there is a legal (shar'i) excuse, it is permissible to perform the prayer sitting. ➐ Performing the prayer sitting earns half the reward, but the Prophet (sallallahu alayhi wa sallam) would receive the full reward even when he performed the prayer sitting.
Source: Muwatta Imam Malik (Narration of Ibn al-Qasim): Commentary by Zubair Ali Zai, Page: 7