Ibrāhīm ibn Ṭahmān, from Manṣūr, from Mujāhid, from Ibn ʿUmar, traced back to the Prophet, with the wording: “In Masjid al-Khayf lies the grave of seventy prophets.”

Al-Haythamī also cited it in Majmaʿ al-Zawāʾid (3 / 298) with the wording: “…the grave of seventy prophets,” and remarked, “Al-Bazzār transmitted it and its narrators are trustworthy.”

That, however, is an incomplete listing, for al-Ṭabarānī likewise recorded it, as you have just seen.

I add: the transmitters in al-Ṭabarānī’s chain are also reliable, with the exception of ʿAbdān ibn Aḥmad—identified as al-Ahwāzī by al-Ṭabarānī in al-Muʿjam al-Ṣaghīr (p. 136)—for whom I could locate no biographical entry. He is not to be confused with ʿAbdān ibn Muḥammad al-Marwazī, another of al-Ṭabarānī’s teachers in the Ṣaghīr (p. 136) and elsewhere; the latter is a trustworthy ḥāfiẓ whose biography appears in Tārīkh Baghdād (11 / 135), Tadhkirat al-Ḥuffāẓ (2 / 230), and other works.

Yet within this chain are narrators known for reporting singular traditions, such as ʿĪsā