What is the preferred length for prayer when the Prophet (peace be upon him) showed variation in lengthening or shortening it?
Chapter on the Times When Prayer is Prohibited
Al-Mughni
Book of Prayer
Primary text
It is obligatory to follow the Prophet (peace be upon him) regarding the lengthening or shortening of the prayer, as he only performs what is superior (Al-Afḍal). Evidence suggests that the preferred action involves an increase in Ruku' (bowing) and Sujud (prostration). This is supported by the statement of Ibn Mas'ud, who said he knew the pairs of chapters the Messenger of Allah (PBUH) used to join in every rak'ah, amounting to twenty chapters from the Mufaṣṣal. This is recorded by Muslim. Furthermore, it is established that every prostration performed by a servant results in Allah recording a good deed, erasing a bad deed, and raising a degree for him.
Supporting text
Some opinions hold that lengthening the prayer is superior, based on the saying of the Messenger of Allah (PBUH): "The best prayer is the longest Qunut," narrated by Muslim. This view is also supported by the fact that the Prophet's (PBUH) most frequent prayer was Tahajjud, which he would lengthen, and one only perseveres in what is best.