What are the limits of prayer, and what is meant by them, based on Ibn al-Qayyim's statement that the one who is negligent in prayer is he who falls short in its ablution, its appointed times, its limits, and its pillars?

Ibn al-Qayyim did not clarify "the boundaries of prayer" in his books. Some scholars have applied this term to the description and form of prayer, and among what has been reported regarding this are:

Al-Jassas's Tafsir: On the shortening of prayer, Ibn Abbas said: "It is only the shortening of the boundaries of prayer, and that you say 'Allahu Akbar,' lower your head, and gesture." Al-Muqaddimat by Ibn Rushd: Shortening the boundaries of prayer means praying by gesturing towards the Qibla or elsewhere during intense fear. Al-Muwafaqat by Al-Shatibi: Ibn Rushd believes that moving the feet during prayer is permissible according to Malik, because it is not among "the boundaries of prayer," meaning it is not something reported from the Prophet, peace be upon him, nor from the predecessors, but rather it is among the innovated matters.