is this authentic  hadith of prophet: "Keep ties with those who cut you off, and do good to those who harmed you, and speak the truth even if it were against your own self."

All praise be to Allah, and may His blessings and peace be on His last messenger, Muhammad,

The wording of the hadeeth mentioned in Arabic is:

"صل من قطعك وأحسن إلى من أساء إليك وقل الحق ولو على نفسك."

This hadeeth was reported by Ibn al-A’rabi and others from Ali (may Allah be pleased with him). It is of controversial authenticity, and although it has been authenticated by as-Soyouti and al-Albani (may Allah bestow mercy on them), it is considered weak by the majority of the scholars of hadeeth, such as Ibn al-Mulaqqin, Ibn Hajar, and others. Although I am not qualified to rate the authenticity of ahadeeth, the arguments of those who considered it weak is stronger.

Having said that, this hadeeth may be reported and acted upon since it is not too weak. Moreover, it is not counter to any Islamic principles or authentic reports. Contarrily, the meaning of this hadeeth is confirmed by the Quran and Sunnah:

Allah said,

"ادْفَعْ بِالَّتِي هِيَ أَحْسَنُ السَّيِّئَةَ نَحْنُ أَعْلَمُ بِمَا يَصِفُونَ."

“Repel, by [means of] what is best, [their] evil. We are most knowing of what they describe.” [Sahih International, 23:96]

Allah also said,

"يا أَيُّهَا الَّذِينَ آمَنُوا كُونُوا قَوَّامِينَ بِالْقِسْطِ شُهَدَاءَ لِلَّهِ وَلَوْ عَلَىٰ أَنفُسِكُمْ أَوِ الْوَالِدَيْنِ وَالْأَقْرَبِينَ ..."

“O you who have believed, be persistently standing firm in justice, witnesses for Allah , even if it be against yourselves or parents and relatives...” [Sahih International, 4:135]

The following hadeeth also corroborates most of the meaning of the one mentioned in the question:

"يَا عُقبَةَ بنَ عَامِر: صِلْ مَنْ قَطَعَكَ، وَأَعْطِ مَنْ حَرَمَكَ، وَاعْفُ عَمَّن ظَلَمَكَ."

The Prophet (peace and blessings of Allah be upon him) said: “O ‘Uqbah ibn ‘Aamir, uphold ties with the one who cuts you off, give to the one who withholds from you, and forgive the one who wrongs you.” Narrated by Ahmad, 4/158; classed as hassan or saheeh or having a good chain (isnad) by al- al-Baihaqi, Mundhiri, al-‘Iraqi, al-Haitami, as well as al-Albaani in al-Silsilah al-Saheehah.

To conclude, there are so many other reports corroborating the same meanings in the hadeeth, which are well established in the revelation and agreed upon as Islamic principles of conduct.

Allah knows best.