Ibn Majah

Benefits and Issues:

➊
The meaning of keeping the back straight in bowing (ruku) and prostration (sujud) is to perform ruku and sujud with tranquility (itmi’nan).
That is, when performing ruku, one should bend fully,
in the correct manner of ruku.
And when performing sujud, one should prostrate with complete tranquility,
in the Sunnah-prescribed manner of sujud.

➋
If the pillars (arkan) of the prayer are not performed with tranquility and moderation, the prayer is not accepted.
For this reason, the Messenger of Allah (sallallahu alayhi wa sallam) ordered that Companion (radi Allahu anhu) to repeat his prayer,
because he had performed the acts of prayer hastily, without tranquility. See: (Sahih al-Bukhari, al-Adhan, Chapter: The Prophet’s (sallallahu alayhi wa sallam) command to the one who does not complete his bowing to repeat [the prayer], Hadith: 793)

1028. Commentary: By straightening or keeping the back straight is meant to have composure (itmi'nan) in bowing (ruku') and prostration (sujud), which according to the hadith is obligatory (wajib). However, the majority of the Hanafis do not consider it necessary, because in the language (lughat), composure is not included in the meaning of bowing and prostration. Can it be asked of these gentlemen: Is the prayer derived from the Qur'an and Sunnah or from the language? Is it not astonishing that if the lexicon states it, it is obligatory, but if it comes in the hadith, it is not obligatory? Astaghfirullah! Inna lillahi wa inna ilayhi raji'un.

1112. Commentary: See Hadith number 1028.

1:
From this it is understood that tranquility (tuma’ninah) and proper arrangement of the pillars (ta‘deel al-arkan) are obligatory in prayer.
As for those who do not consider this obligatory, they argue that this would entail an addition to the explicit text, because in the Noble Qur’an the command is for prostration (sujud) in an unrestricted sense, and tranquility is not included in that; such an addition is not permissible.
The response to this is that this is not an addition, rather it is a clarification of the meaning of prostration—that what is intended is not the linguistic prostration, but the legal (shar‘i) prostration, whose concept includes tranquility as well.

Benefit:
Prayer should be performed with complete tranquility and composure. A person carries out worldly tasks with care and order, but when it comes to prayer and similar acts, he hastily completes them just to get them over with; this should not be done. A person should worship Allah as well, in accordance with the Qur'an and Hadith, observing full etiquette and respect, because after correct belief, the foundation of both this world and the Hereafter rests upon it.