What is the meaning of Abu Humayd al-Sa’idi’s statement in describing the Prophet’s prayer, peace and blessings be upon him: "He lowers himself, separating his hands from his sides, then prostrates"? Is the phrase "then prostrates" considered an anomalous addition that is not authentic due to its absence in other narrations?

What you have mentioned is part of the description of the Prophet's prayer (peace be upon him). The phrase "then he prostrates" appeared in "Muntaqa Ibn al-Jarud," "Sahih (definition) Ibn Khuzaymah," and other sources. It was narrated in the Hadith (definition) of Abu Humayd al-Sa'idi: "Then he descends to the ground, separating his hands from his sides." Muhammad ibn Yahya added: "Then he prostrates." And the Companions confirmed this. In "Sunan Ibn Majah": "Then he descends to the ground, and separates his hands from his sides, then he raises his head... then he prostrates." The meaning of "separates his hands from his sides" is to distance his elbows from his sides during prostration.