Saying Tehilim at the conclusion of the evening prayer:
At the conclusion of Maariv it is accustomed for the congregation to recite the entire book of Tehilim.
 Otzer Minhagei Chabad p. 212 and so is the custom in the Beis Midrash of the Rebbe.
Saying Tehilim after Maariv together with the congregation was an institution of the Rebbe Rayatz to the Temimim, brought in Hakeria Vehakedusha Erev Rosh Chodesh Teives 1941. It is also mentioned in the Tur 579; Darkei Moshe 619/4; Shlah Miseches Yuma 229/1