Is the Hadith "O Allah, do not forgive Muhallam ibn Juthamah" authentic?

The gist of the answer: Ahmad, Abu Dawud, and Ibn Majah narrated the hadith of Muhallam ibn Jaththāmah al-Laythi, who killed a man from Ashja', which led to animosity and the intervention of 'Uyaynah and al-Aqra'. The Messenger of Allah, peace be upon him, offered the blood money twice to 'Uyaynah, but he refused. A man from Banu Layth intervened and said, "Enact a precedent today and change it tomorrow." The Messenger of Allah, peace be upon him, then said, "Fifty [camels] in our immediate presence, and fifty when we return to Medina." Muhallam came repentant, and the Messenger of Allah, peace be upon him, loudly said to him, "O Allah, do not forgive Muhallam." Then it was said that the Messenger of Allah, peace be upon him, later sought forgiveness for him.

Al-Albani and Shu'ayb al-Arna'ut declared this hadith weak due to the unknown status (jahalah) of Ziyad ibn Dhamrah.