Have not come across this as a Hadith of Rasulullah (sallallahu ‘alayhi wa sallam).

Imam Bayhaqi (rahimahullah) has recorded the following narration as the statement of Hamdun ibn Ahmad Al Qassar (rahimahullah) who says: “When one of your brothers slips up [commits an error], then seek seventy excuses for him. If your hearts do not accept it, then know that the fault is with your own self”.(Shu’abul Iman, Hadith: 10684)

Upon further research, it appears that this quote is not an authentic hadith, which means it cannot be attributed to the Prophet Muhammad. The greatest evidence of the quote's origins traces back to Hamdun al-Qassar, one of the great early Muslims (d. late 9th century C.E.). It is reported that he said,

“If a friend among your friends errs, make seventy excuses for him. If your hearts are unable to do this, then know that the shortcoming is in your own selves.”

While not Prophetic advice, this should still be considered good, sound advice for any Muslim. While he didn't use these exact words, the Prophet Muhammad did advise Muslims to cover up the faults of others. The practice of making 70 excuses helps one to become humble and to be forgiving. In doing so, we recognize that only Allah sees and knows all things, even the secrets of the hearts

This narration should be quoted as recorded as the statement.

Note - This statement should not confuse with this following hadith. 

Abdullah ibn Umar reported: A man came to the Prophet, peace and blessings be upon him, and he said, “O Messenger of Allah, how many times should I pardon my servant?” The Prophet said, “Seventy times in each day.” Source: Sunan al-Tirmidhī 1949

 7 fatwa links below

https://hadithanswers.com/seek-seventy-excuses-for-your-brother/
https://islamqa.info/en/answers/128159/how-sound-is-the-hadeeth-seek-seventy-excuses-for-your-brother
https://www.learnreligions.com/70-excuses-2004271
https://seekersguidance.org/articles/general-artices/making-70-excuses-for-others-in-islam-a-key-duty-of-brotherhood/
https://islamqa.org/hanafi/qibla-hanafi/35717/re-70-excuses/
https://ussunnah.org/2020/10/21/making-excuses-for-your-brother
https://www.islamweb.net/en/fatwa/91377/seek-for-your-brother-seventy-excuses

And Allah Ta’ala Knows best.