Sunnah.com reference : Book 7, Hadith 77English translation : Book 7, Hadith 845Arabic reference : Book 7, Hadith 842
Narrated Abu Umamah (RA):
The Prophet (ﷺ) said: "Whoever intercedes for his brother and that one gives him a gift for that (intercession) which he accepts, he has engaged in one of the most terrible types of Riba (undeserving increase in something)."

[Reported by Ahmad and Abu Dawud, but one of its narrators' reliability has been doubted].
وَعَنْ أَبِي أُمَامَةَ ‏- رضى الله عنه ‏- عَنِ اَلنَّبِيِّ ‏- صلى الله عليه وسلم ‏-قَالَ: { مَنْ شَفَعَ لِأَخِيهِ شَفَاعَةً, فَأَهْدَى لَهُ هَدِيَّةً, فَقَبِلَهَا, فَقَدْ أَتَى بَابًا عَظِيماً مِنْ أَبْوَابِ اَلرِّبَا }  رَوَاهُ أَحْمَدُ, وَأَبُو دَاوُدَ, وَفِي إِسْنَادِهِ مَقَالٌ  1‏ .‏‏1 ‏- ضعيف.‏ رواه أحمد ( 5 / 261 )‏، وأبو داود ( 3541 )‏.‏