Halacha: Don't Sit

DON'T SIT

The Shulchan Aruch (1) rules that it's forbidden to sit within 4 Amot (6 or 8ft) of one praying Shmonei Esreh. This holds true in front, on the sides, or in back of the one praying.

The same applies if the one who is praying is finished with the text of Shmonei Esreh, and is reciting his own personal requests (2). Some are lenient in a time of pressing need (3).

If someone didn't take 3 steps back yet, because there's someone behind him still praying, one may sit with his 4 amot (4).

The reason for the prohibition is, the 4 amot where one's praying Amidah is very holy. Therefore one who sits and isn't involved in a very holy activity, is acting in a disgracing manner by showing there isn't any holiness there (5).

The Mishna Berurah explains the prohibition is that it shows one isn't also accepting upon himself the yoke of heaven (6). Most explain the 2 opinions to be one in the same. (7)

The Shulchan Aruch (8) also states that if someone is involved in prayer, one may keep seated or sit within the 4 amot of one praying Amidah.

The Shulchan Aruch (9) says that if one was already seated and someone just came into your 4 amot and started praying Amidah, one does not have to move, as he came into your boundary.

There are some who suggest, out of an act of piety to still distance oneself (10).

Sources:
(1) O"C 102:1
(2) M"B 102:1:3
(3) Yalkut Yosef 102:19
(4) Ohr L'Tzion Chelek 2 Ch.7 Halacha 23, Yalkut Yosef Halacha 20
(5) Tur, Beit Yosef, Taz (s'k 3)
(6) M"B 102:1:6
(7) Because of M"B 102:1:14
(8) O"C 102:1
(9) O"C 102:3
(10) Rema 102:3