What is the source for saying Shema Yisrael prior to death? Is this a Halacha?

Quite interestingly, we do not find any source in Jewish literature that speaks of an obligation for saying Shema Yisrael prior to death. It is however recorded in the Sefer Mavor Yabok as part of the prayers to be said prior to death to help guard the soul, and is seemingly the one and only source on this matter. Practically, this is a custom widespread amongst all Israel, even those who are not observant, and is seemingly an instinctive call of the soul prior to death which is above and beyond any instruction or ruling that needs to be recorded. To quote from a talk of the Rebbe Rayatz, “Prior to going to sleep one is to read the Shema just as prior to the exit of one’s soul from this world to the next world one is to say the Shema.” It’s interesting to note that while not an actual source for the obligation, the concept of dying with Shema on one’s lips is famously recorded in the Talmud regarding rebbe Akiva who was being tortured while he was saying the Shema and his soul left his body when he said the word Echad. Likewise, the Talmud states that prior to Yaakov’s death his children said Shema Yisrael in reply to his question of their faithfulness. It is likewise recorded the Yaakov recited the verse of Shema when he met his son Joseph for the first time in Egypt.

See Bereishis 46:29 and Rashi there; Pesachim 56a; Brachos 61b; Mavor Yabok Sifsei Tzedek 7 p. 132; Sefer Hamamarim 5701 p. 158; Darkei Chesed p. 259; Nitei Gavriel 3:14 footnote 24