One should not recite Tehillim during his tefillah after he has said ‘Baruch She’amar’.
Sources and Explanations
Rabbi Chaim Palagi brings this in his Kaf HaChaim (siman 14, ot 15): “If there is no minyan in the synagogue, one should not recite mizmorim of Tehillim in the interim once he has already begun Baruch She’amar, and needless to say before ‘Vayevarech David’, as the Rav Petach HaDevir wrote (siman 65, ot 3). According to this, one must rebuke someone who recites [them] while the maftir prolongs the melody of the kaddish; and even during Barechu they did not permit saying any pasuk (verse) when he is in Yotzer, as is known from the books of the tzaddikim.” In the notes they wrote that perhaps he is referring to what the Rema wrote (Shulchan Aruch, Orach Chaim 57:1): “The custom is that the chazzan prolongs Barechu, and the congregation says ‘Yitbarach veyishtabach’ etc. while he is prolonging Barechu.” And in Petach HaDevir (ot 3) he wrote that his heart is hesitant about this because of interruption, and therefore he refrains from saying it at Shacharit.
From the sefer “Shaf Veyativ – Laws of Saying Tehillim” by Rabbi Shay Tahan, Chapter 7: What Is Forbidden and Permitted When Saying Tehillim, Halacha 12 (p. 127 in the print edition). Translated from the Hebrew original.
