The Muḥaddith, al-Albānī [d. 1419 AH], may Allāh have mercy upon him, wrote:¹

6. A person must stand in prayer, it is a rukn for all except:

i. Those engaged in ṣalāt al-khawf² and intense combat. Under such circumstances, they are permitted to pray while riding.

ii. The indisposed who are not able to stand. They are to pray seated while they can. If not, they are to recline on one side.

iii. Those offering al-nawāfil. They either pray while riding or if they prefer while sitting. They bow and prostrate by gesturing with their head, making the sujūd lower than the rukūʿ. This applies to the ill as well.

7. A person who prays in a sitting posture is not permitted to place an elevated object on the ground to prostrate upon it; but in the event he cannot directly touch the ground with his forehead, he is to make his sujūd lower than his rukūʿ as above-mentioned.

Prayer Abroad a Ship or Plane

8. It is permissible to perform the obligatory prayers abroad on a ship and airplane.

9. It is permissible to perform them while seated should one fear collapsing.

10. It is permissible, while standing, to lean against a pillar or cane if one is afflicted with old age or corporeal infirmity.

Standing and Sitting Postures

11. Night prayer may be observed while standing or sitting (or by combining both postures) without providing any justification for the latter. So, a person may pray and recite while seated, and muʿmīnts before the rukūʿ, he may rise and recite the remaining verses, then bow and prostrate. The following rakʿah is observed in a similar fashion.

12. If a person prays seated, he may do so cross-legged, or he may adopt any other sitting posture that is agreeable to him.

Endnotes:

[1] Al-Albānī [d. 1419 AH], Talkhīs Ṣifat Ṣalāt al-Nabī, (ﷺ)  [al-Maktab al-Islāmī, 5th ed. (1404), 8-9].
[2] The Fear Prayer.