In Gemara Shevuot 15b, they explain how is the process to extend the surface of Yerushalaim and its Qedusha. One of the things is to recite the Psalm 91 up to the word Machsi (see hagahot Hagra, see Abudarham who says that they are 60 words) and an other mizmor (a part of Tehilim 3). Subsequently, the Gemara says that Rabbi Yehoshua Ben Levi said those Psalms when he gets to sleep.
In Bet Yosef OC 239, this Gemara is quoted. But in SA he doesn't write to stop the Mizmor after the word Machsi. I checked the Nuschaot of QS Al Hamita for Sefaradim and Ashkenazim. Sefaradim make as it's written in Gemara and Ashkenazim recite the whole Mizmor.
What is the source of those who don't follow the Gemara?