The Laws of Prayer (The morning services) (42)

1. One may not sit within four cubits of one who is praying the Amidah. This applies whether one is in front of him, behind him or to one side. Even if he has already finished the main part of the Amidah and is saying the supplications at the end of the Amidah one may not sit within four cubits of him. (Shulchan Aruch and Mishne Berurah 102)

2. Not only is sitting down forbidden, but even leaning is forbidden as one must stand properly. In case of necessity, one may allow a slight leaning, in a way that if the supporting thing would be removed, that he would not fall. (ibid)