The Preferred Method of Performing Salah whilst Sitting on a Chair

Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him) said:

The one who prays sitting on the ground or on a chair must make his prostration lower than his bowing. The Sunnah is for him to put his hands on his knees when bowing, and when prostrating the Sunnah is to put them on the ground if possible; if he cannot do that then he should put them on his knees, because it was proven that the Prophet (peace and blessings of Allaah be upon him) said: “I have been commanded to prostrate on seven bones: the forehead – and he pointed to his nose – she two hands, the two knees, and the edges of the two feet (i.e., the toes).”

f a person is unable to do that and prays on a chair, there is nothing wrong with that, because Allaah says (interpretation of the meaning):
o keep your duty to Allaah and fear Him as much as you can”

l-Taghaabaun 64:16

Fataawa Ibn Baaz, 12/245, 246

With regard to the placement of the chair in the row, the scholars (may Allaah have mercy on them) stated that the one who prays sitting down should have his posterior in line with the row when he is sitting, so he should not be in front of the row or behind it at that point, because that is the place in which the body settles.
See Asnaa al-Mataalib, 1/222; Tuhfat al-Muhtaaj, 2/157; Sharh Muntaha al-Iraadaat, 1/279

Shaykh Ibn ‘Uthaymeen said:

If a person cannot pray standing, he should pray sitting, but it is better to sit cross-legged in the place of standing and bowing.

From his essay, Tahaarat al-Mareed wa Salaatuhu

This sitting cross-legged is not obligatory; he may sit however he wants because the Prophet (peace and blessings of Allaah be upon him) said: “If he cannot then sitting,” and he did not explain how he should sit.

See al-Sharh al-Mumti’, 4/462.