This hadeeth was narrated by Ibn Shaheen in at-Targheeb fi Fadaa’il al-A‘maal (p. 14) via ‘Awn ibn ‘Umaarah, from Sakan al-Burjumi, from Hajjaaj ibn Sinaan, from ‘Ali ibn Zayd, from Sa‘eed ibn al-Musayyib, from Abu Hurayrah, who said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Sending blessings upon me will be light on the siraat. Whoever sends blessings upon me eighty times on Friday, his sins will be forgiven for eighty years.” 

Some of the later scholars were lenient and classed this hadeeth as hasan; however the correct view is that the hadeeth is da‘eef (weak), and in fact it is da‘eef jiddan (very weak), because its isnaad includes three da‘eef narrators, namely: 

1. ‘Ali ibn Zayd ibn Jad‘aan al-Basri 

Hammaad ibn Zayd said: He used to mix up hadeeths. Shu‘bah stated that he was confused. Ahmad said: He is nothing. Abu Zar‘ah said: He is not strong; he made mistakes. Abu Haatim said: He cannot be quoted as evidence. Ad-Daaraqutni said: In my view there is still some weakness in him

End quote from al-Mughni fi ad-Du‘afa’ by adh-Dhahabi (2/447) 

2. Hajjaaj ibn Sinaan 

Al-Azdi said concerning him: He is matrook (rejected). See: Lisaan al-Mizaan by Ibn Hajar (2/563). 

3. ‘Awn ibn ‘Umaarah al-Qaysi 

Abu Zar‘ah said: His hadeeth is odd. Al-Haakim said: I met him but I did not write down anything from him, and his hadeeth was odd and he was weak in narrating hadeeth. Abu Dawood said: He is da‘eef.

End quote from Tahdheeb at-Tahdheeb (8/173). 

Among the imams (leading scholars) who classed the hadeeth as da‘eef were: ad-Daaraqutni; al-Haafiz ibn Hajar in Nataa’ij al-Afkaar (5/56)’ as-Sakhkhaawi in al-Qawl al-Badee‘ (p.284); al-Mannaawi in Fayd al-Qadeer (4/249); al-Hoot al-Beiruti in Asna al-Mataalib (p. 175); Shaykh al-Albaani in al-Silsilah ad-Da‘eefah (8/274). 

There is a corroborating report for the hadeeth in the hadeeth of Anas ibn Maalik, who said: I was standing in front of the Messenger of Allah (blessings and peace of Allah be upon him) and he said: “Whoever sends blessings upon me on Friday eighty times, Allah will forgive him the sins of eighty years.” It was said: How may we send blessings upon you, O Messenger of Allah? He said: “Say: ‘O Allah, send blessings upon Muhammad, Your slave, Your Messenger and Your Prophet, the unlettered Prophet’ and count it as one.”

Narrated by al-Khateeb al-Baghdaadi in Tareekh Baghdad (13/464). Its isnaad includes Wahb ibn Dawood ibn Sulaymeen ad-Dareer.

Adh-Dhahabi said: al-Khateeb said: He was not trustworthy. Then he mentioned a hadeeth fabricated by him.

End quote from al-Mughni fi ad-Du‘afa’ (2/727) 

Something similar was mentioned by Ibn al-Jawzi in al-Ahaadeeth al-Waahiyah (no. 796 – where he listed very weak hadeeths). 

Shaykh al-Albaani said: He should have listed it in his other book, al-Ahaadeeth al-Mawdoo‘aat (Fabricated Hadeeths). That would have been more appropriate, because tThe signs of fabrication are very clear in it. The saheeh hadeeths that speak of the virtue of sending blessings upon the Prophet (blessings and peace of Allah be upon him) are sufficient and there is no need for reports such as this. For example, the Prophet (blessings and peace of Allah be upon him) said: “Whoever sends blessings upon me once, Allah will send blessings upon him tenfold.” Narrated by Muslim and others. End quote. Silsilat al-Ahaadeeth ad-Da‘eefah wa al-Mawdoo‘ah (1/383) 

And Allah knows best.