Gemara:

Rosh Hashana 

Halacha:

Tefillah 

Collections: R' Grunstein Shir Shel Yom at Mincha

References: Rosh Hashana: 31A  

Description

While some feel that שיר של יום is not מעכב/doesn't invalidate the afternoon sacrafice [and thus, we don't HAVE to say it], others feel it's so crucial that we dare not make a mistake [and thus omit it to be on the side of caution