Ibn Majah

Commentary:
(1)
This narration of Ibn Umar radi Allahu anhuma is weak through this chain, however, its being hasan (good) is established through other routes.
Isa Khayyat, who is also called Hannaṭ, see: (Taqrib al-Tahdhib: 5352),
has indicated towards the hadith of Abu Hurairah radi Allahu anhu, and most likely, by this is meant the hadith in Sahih Muslim in which Abu Hurairah radi Allahu anhu narrated the statement of the Prophet sallallahu alayhi wa sallam:
“When any person sits for relieving himself, he should neither face the qiblah nor turn his back towards it.” (Sahih Muslim, al-Taharah, Chapter: al-Istinja, Hadith: 265)
And the hadith of Ibn Umar radi Allahu anhuma is the very one under discussion, in which permission for this is understood within a four-walled enclosure.
In addition, Hafiz Ibn Hajar rahimahullah has clarified that Ibn Umar radi Allahu anhuma climbed onto the roof for some purpose, and his gaze suddenly fell upon the Prophet sallallahu alayhi wa sallam; he did not look intentionally, but from this, the legal ruling was suddenly learned that doing so within a four-walled enclosure is permissible. (al-Mawsu‘ah al-Hadithiyyah: 8/616)

(2)
Allamah Waheed al-Zaman has stated that from the Prophet sallallahu alayhi wa sallam, there are narrations both regarding facing the qiblah and turning one’s back towards it, as well as narrations prohibiting it.
There can be two ways to reconcile between them:
First, that the prohibition is of a binding (tahrimi) nature, but the prohibition is only in the desert and open places, while within a building it is permissible.
The second case is that the prohibition is considered of a non-binding (tanzihi) nature, so avoidance is preferable, and from the action of the Prophet sallallahu alayhi wa sallam, permissibility is established. (Translation of Sunan Ibn Majah by Allamah Waheed al-Zaman.
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with adaptation)