Du’a of a Sahabi During His Salah

Is the du’a authentic?

Hanzalah ibn ‘Ali narrated that:

Mihjan ibn Al-Adra narrated to him that the Messenger of Allah (sallallahu ‘alayhi wa sallam) entered the masjid and there was a man who had finished his prayer and he was reciting the tashahhud [after which] he said:

اللّٰهُمَّ إِنِّي أَسْأَلُكَ يَا اللهُ بِأَنَّكَ الْوَاحِدُ الأَحَدُ الصَّمَدُ الَّذِي لَمْ يَلِدْ وَلَمْ يُولَدْ وَلَمْ يَكُنْ لَهُ كُفُوًا أَحَدٌ أَنْ تَغْفِرَ لِي ذُنُوبِي إِنَّكَ أَنْتَ الْغَفُورُ الرَّحِيمُ

“Allahumma inni as aluka ya Allah! Bi annaka wahidul ahadus samad, aladhi lam yalid wa lam yuwlad, wa lam yakul lahu kufuwan ahad, an taghfirali zunubi, innaka antal ghafurur rahim”

Translation

O Allah, I ask of You, O Allah, as You are the One, the Only, the Self-Sufficient Master, Who begets not nor was He begotten, and there is none equal or comparable to Him, forgive me my sins, for You are the Oft-Forgiving, Most Merciful.

The Messenger of Allah (sallallahu ‘alayhi wa sallam) said three times: “He has been forgiven.”