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Abu Umama reported God’s Messenger as saying, “If anyone intercedes for, someone and that one gives him for it a present which he accepts, he has been guilty of a serious type-of usury.” Abu Dawud transmitted it.
Source: sunnah.com
Other translations
Arabic
عَنْ أَبِي أُمَامَةَ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «مَنْ شَفَعَ لِأَحَدٍ شَفَاعَةً فَأَهْدَى لَهُ هَدِيَّةً عَلَيْهَا فَقَبِلَهَا فَقَدْ أَتَى بَابًا عَظِيمًا مِنْ أَبْوَابِ الرِّبَا» . رَوَاهُ أَبُو دَاوُد
sunnah.com