A man was mentioned before the Prophet ﷺ and another man praised him greatly The Prophet ﷺ said, "May Allah's Mercy be on you ! You have cut the neck of your friend." The Prophet ﷺ repeated this sentence many times and said, "If it is indispensable for anyone of you to praise someone, then he should say, 'I think that he is so-and-so," if he really thinks that he is such. Allah is the One Who will take his accounts (as He knows his reality) and no-one can sanctify anybody before Allah." (Khalid said, "Woe to you," instead of "Allah's Mercy be on you.") (Using translation from Bukhārī 6061)   

أَنَّ رَجُلًا ذُكِرَ عِنْدَ النَّبِيِّ ﷺ فَأَثْنَى عَلَيْهِ رَجُلٌ خَيْرًا فَقَالَ رَسُولُ اللهِ ﷺ وَيْحَكَ قَطَعْتَ عُنُقَ صَاحِبِكَ يَقُولُهُ مِرَارًا إِنْ كَانَ أَحَدُكُمْ مَادِحًا أَخَاهُ لَا مَحَالَةَ فَلْيَقُلْ أَحْسِبُ كَذَا وَكَذَا إِنْ كَانَ يَرَى أَنَّهُ كَذَاكَ وَحَسِيبُهُ اللهُ وَلَا يُزَكِّ أَحَدٌ عَلَى اللهِ  

Woe be to thee, you have broken the neck of your friend, you have broken the neck of your friend-he said this twice. If one of you has to praise his friend at all, he should say: I think (him to be) so and Allah knows it well and I do not know the secret of the heart and Allah knows the destined end, and I cannot testify his purity against Allah but (he appears) to be so and so. (Using translation from Muslim 3000a)   

مَدَحَ رَجُلٌ رَجُلًا عِنْدَ النَّبِيِّ ﷺ فَقَالَ وَيْلَكَ قَطَعْتَ عُنُقَ صَاحِبِكَ مِرَارًا إِذَا كَانَ أَحَدُكُمْ مَادِحًا صَاحِبَهُ لَا مَحَالَةَ فَلْيَقُلْ أَحْسِبُ فُلَانًا وَاللهُ حَسِيبُهُ وَلَا أُزَكِّي عَلَى اللهِ أَحَدًا أَحْسِبُهُ إِنْ كَانَ يَعْلَمُ ذَاكَ كَذَا وَكَذَا  

You killed him, or you sliced the back of a person. (Using translation from Muslim 3001)   

سَمِعَ النَّبِيُّ ﷺ رَجُلًا يُثْنِي عَلَى رَجُلٍ وَيُطْرِيهِ فِي الْمِدْحَةِ فَقَالَ لَقَدْ أَهْلَكْتُمْ أَوْ قَطَعْتُمْ ظَهْرَ الرَّجُلِ  

"A man stood and praised one of the 'Amirs so Al-Miqdad bin Al-Aswad threw dust in his face and said: 'The Messenger of Allah ﷺ ordered us to throw dust in the faces of those who praise others.'" (Using translation from Tirmidhī 2393)   

قَامَ رَجُلٌ فَأَثْنَى عَلَى أَمِيرٍ مِنَ الْأُمَرَاءِ فَجَعَلَ الْمِقْدَادُ يَحْثُو فِي وَجْهِهِ التُّرَابَ وَقَالَ أَمَرَنَا رَسُولُ اللهِ ﷺ أَنْ نَحْثُوَ فِي وُجُوهِ الْمَدَّاحِينَ التُّرَابَ  

The Prophet ﷺ said, "Truthfulness leads to righteousness, and righteousness leads to Paradise. And a man keeps on telling the truth until he becomes a truthful person. Falsehood leads to Al-Fajur (i.e. wickedness, evil-doing), and Al-Fajur (wickedness) leads to the (Hell) Fire, and a man may keep on telling lies till he is written before Allah, a liar." (Using translation from Bukhārī 6094)   

قَالَ رَسُولُ اللهِ ﷺ إِنَّ الصِّدْقَ يَهْدِي إِلَى الْبِرِّ وَإِنَّ الْبِرَّ يَهْدِي إِلَى الْجَنَّةِ وَإِنَّ الرَّجُلَ لَيَصْدُقُ حَتَّى يُكْتَبَ عِنْدَ اللهِ صَدِّيقًا وَإِنَّ الْكَذِبَ يَهْدِي إِلَى الْفُجُورِ وَإِنَّ الْفُجُورَ يَهْدِي إِلَى النَّارِ وَإِنَّ الرَّجُلَ لَيَكْذِبُ حَتَّى يُكْتَبَ عِنْدَ اللهِ كَذَّابًا