Grading: This hadith is sahih, and this chain is hasan in corroborating chains and supporting reports because of Abdullah bin Salih (who is the Egyptian, the scribe of al-Layth), and he has been corroborated.
Technical note / Defect ('illah): Al-Qasim is "Ibn Abd al-Rahman Abu Abd al-Rahman al-Dimashqi".

Note / Clarification: Abu Umamah: he is "Sudayy bin Ajlan al-Bahili".
Reference / Source: Al-Tabarani recorded it in "al-Kabir" (7757) and in "Musnad al-Shamiyyin" (1941) from Bakr bin Sahl al-Dimyati, and al-Bayhaqi in "Bayan Khata' man Akhta'a ala al-Shafi'i" (pp. 301-302) by way of Muhammad bin Ishaq al-Saghani, and both of them (Bakr and al-Saghani) narrated it from Abu Salih Abdullah bin Salih with this same chain.
Technical note / Defect ('illah): However, Bakr said in it "Mu'awiyah bin Salih from Kathir bin al-Harith", and al-Saghani said "Mu'awiyah from Yahya bin al-Harith". All three of these narrators (al-Ala' bin al-Harith, Kathir bin al-Harith and Yahya bin al-Harith) are ones from whom Mu'awiyah bin Salih narrates, but this disagreement in identifying the narrator of this report is in fact due to the idtirab (inconsistency) of Abdullah bin Salih, because there was a defect in his memory.

Important point: The mahfuz (preserved, correct) name in it is "al-Ala' bin al-Harith", as occurs in the narration of al-Sha'rani with the author (al-Hakim).
Reference / Source: Likewise Ma'n bin Isa al-Qazzaz (who is one of the trustworthy and reliable narrators) narrated it from Mu'awiyah bin Salih, as in Ibn Adi's "al-Kamil" (6/405-406) and al-Quda'i's "Musnad al-Shihab" (901).
Grading: The chain up to him is sahih; al-Ala' bin al-Harith is thiqah, and likewise Mu'awiyah and al-Qasim are thiqah, so this chain is established as sahih. And Allah knows best.

Reference / Source: Al-Tabarani recorded it in "al-Kabir" (7894) by way of Abu Abd al-Malik, from al-Qasim, likewise.
Technical note / Defect ('illah): This Abu Abd al-Malik is in fact "Ali bin Yazid al-Alhani", and he is a weak narrator.

Corroborating chains and supporting reports: This hadith has as a shahid (supporting report) the hadith of Anas bin Malik, which will come ahead at number (8567), and its chain is weak.

Corroborating chains and supporting reports: Another supporting report is the hadith of Mu'awiyah bin Abi Sufyan, which is in al-Tabarani's "al-Kabir" 19/(835) and al-Bayhaqi's "Bayan Khata' man Akhta'a ala al-Shafi'i" (p. 301), and its narrators are trustworthy.

Corroborating chains and supporting reports: A third supporting report is the hadith of Imran bin Husayn, which is in Abu Nu'aym's "al-Hilyah" (7/262), and in it is a narrator whom we could not identify.

Corroborating chains and supporting reports: And for his (peace and blessings be upon him) words: "The Hour will not be established except upon the worst of creation", there are several supporting reports in wording and in meaning, the most famous of which is the hadith of Abdullah bin Mas'ud (may Allah be pleased with him), which is in Sahih Muslim at number (2949) with these words: "لا تقوم الساعة إلا على شرار الناس" (The Hour will not be established except upon the worst of people).