What is the ruling on kneeling during prayer when sitting for the Tashahhud and between the two prostrations for one who cannot bend their knee or finds difficulty in doing so?

Kneeling (juthuw) is a specific posture of sitting, usually adopted in situations of dispute, fear, or urgency, as opposed to the customary sitting in comfortable circumstances. The sunnah posture for sitting between the two prostrations and in the first tashahhud is al-iftirash (sitting on the left foot with the right foot upright), and in the final tashahhud it is at-tawarruk (sitting on the ground with both feet out to the right).

If a worshipper is unable to sit in the sunnah posture due to an excuse, it is permissible for him to sit in whatever way is easy for him, such as kneeling on his knees. This is based on the Almighty's saying: ﴿So fear Allah as much as you are able﴾, and the Prophet's (peace be upon him) saying: "When I command you to do something, do as much of it as you are able," and his (peace be upon him) saying to Imran ibn Husayn: "Pray standing; if you are unable, then sitting; if you are unable, then on your side." The jurists have stated that the manner of sitting in prayer does not require a specific posture for validity; however the worshipper sits, it is sufficient.