Death in the Afternoon is mentioned in Melachim Beis 4:20:
וַיִּשָּׂאֵ֔הוּ וַיְבִיאֵ֖הוּ אֶל־אִמּ֑וֹ וַיֵּ֧שֶׁב עַל־בִּרְכֶּ֛יהָ עַד־הַֽצָּהֳרַ֖יִם וַיָּמֹֽת׃
And they carried him and they brought him to his mother, and he laid on her lap until noon, and he died.
Now, it’s a debate whether “until” includes the number specified (עד ועד בכלל או לא עד בכלל). For instance, R’ Yehudah (Berachos 27a), Rebbi (Erchin 18a-b), and maybe R’ Yochanan (Nazir 6b) hold that it is included, while the Chachamim (Erchin 18a-b) and maybe R’ Yoshaya (Nazir 6b) hold that it is not included.
If you hold עד ועד בכלל, then the child was unconscious until noon and died after - thus, death in the afternoon. If you hold לא עד בכלל, then the child died exactly at noon.
Now, there is some confusion as to whether AM or PM should be used for 12 noon. Therefore, we can pose a Sefeik Sefeikah: maybe we hold עד ועד בכלל, and even if we don’t, maybe noon is considered afternoon anyway. This is even reversible: maybe noon is considered afternoon, and even if it’s not, maybe we hold עד ועד בכלל.
So at least we can conclude that Death in the Afternoon is mentioned in Tanach. As for those who hold עד ולא עד בכלל and that noon is AM...? אין מקשין על האגדה.