The truest word spoken by an Arab (pre-Islamic) in poetry is this verse of Labid: "Behold! apart from Allah everything is vain."  

عَنِ النَّبِيِّ ﷺ قَالَ أَشْعَرُ كَلِمَةٍ تَكَلَّمَتْ بِهَا الْعَرَبُ كَلِمَةُ لَبِيدٍ أَلاَ كُلُّ شَىْءٍ مَا خَلاَ اللَّهَ بَاطِلٌ  

that the Prophet ﷺ said: "The best statement spoken by the Arab is the saying of Labid: 'Everything aside from Allah perishes.'"  

عَنِ النَّبِيِّ ﷺ قَالَ أَشْعَرُ كَلِمَةٍ تَكَلَّمَتْ بِهَا الْعَرَبُ كَلِمَةُ لَبِيدٍ أَلاَ كُلُّ شَيْءٍ مَا خَلاَ اللَّهَ بَاطِلُ  

[Machine] The Messenger of Allah ﷺ said, "I perceive a profound saying that the Arabs have uttered, the saying of Labid [Al-Bahr Al-Taweel]: 'Verily, everything that is other than Allah is futile.'"  

قَالَ رَسُولُ اللهِ ﷺ أَشْعَرُ كَلِمَةٍ قَالَتْهَا الْعَرَبُ كَلِمَةُ لَبِيدٍ[البحر الطويل]أَلَا كُلُّ شَيْءٍ مَا خَلَا اللهَ بَاطِلُ  

[Machine] On the authority of the Messenger of Allah ﷺ , he said, "I feel a house." The Arabs used to say it as a word of Labid.  

عَنْ رَسُولِ اللَّهِ ﷺ قَالَ أَشْعَرُ بَيْتٍ قَالَتْهُ الْعَرَبُ كَلِمَةُ لَبِيدٍ  

The truest word spoken by an Arab (pre-Islamic) in poetry is this verse of Labid: "Behold! apart from Allah everything is vain." (Using translation from Muslim 2256a)   

"أَشْعَرُ كَلَمةٍ تَكلَّمَتْ بِهَا الْعَرَبُ كلِمَةُ لَبيدٍ: أَلا كُلَّ شيءٍ مَا خَلا الله بَاطِلٌ" .