ENOfficial
‘Amr b. Shu'aib, on his father’s authority, told that his grandfather reported God’s Messenger as saying in the course of his sermon, "Observe fully the alliance made in the pre-Islamic period, for it, i.e. Islam, will only make it stronger; but make no new alliances in Islam.” The source is not given. See Tirmidhi, Siyar, 30
Source: sunnah.com
Other translations
Arabic
وَعَنْ عَمْرِو بْنِ شُعَيْبٍ عَنْ أَبِيهِ عَنْ جَدِّهِ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ فِي خطْبَة: «أَوْفوا بِحلف الْجَاهِلِيَّة فَإِنَّهُ لَا يزِيد يَعْنِي الْإِسْلَامَ إِلَّا شِدَّةً وَلَا تُحْدِثُوا حَلِفًا فِي الإِسلامِ» . رَوَاهُ الترمذيُّ من طريقِ ابنِ ذَكْوَانَ عَنْ عَمْرٍو وَقَالَ: حَسَنٌ.
sunnah.com