How authentic is the Hadith: "O Allah, do not forgive Muhallam bin Jaththamah"?

The Hadith which mentions that the Messenger of Allah, peace and blessings be upon him, said: "O Allah, do not forgive Muhallam" has a weak chain of narration due to the ignorance of Ziyad ibn Dhamrah. As for the authentic narration, it indicates that the verse of clarification (Ayat al-Tabyyun) in Surat an-Nisa (verse 94) was revealed concerning the incident of Muhallam ibn Jaththama killing Amir al-Ashja'i. Even if the first Hadith were authentic, it would not mean that Muhallam was not forgiven, especially after his remorse and repentance. It has been reported in some narrations that the Messenger of Allah, peace and blessings be upon him, later sought forgiveness for him.