It might be that you are referring to what Ibn Hajar mentioned in At-Talkhees Al-Habeer, as he said: "The Hadeeth that a delegation came to the Prophet and with them was an attractive boy, so he [the Prophet] made him sit behind him and said: “I fear what happened to my brother Daawood”; Ibn As-Salah said: “This is a Dha’eef (weak) and baseless Hadeeth.” Ibn Shaaheen reported it in Al-Afraad from Mujaalid from Ash-Sha’bi who said: “A delegation from ‘Abd Al-Qays came to the Prophet and with them was an attractive, beardless boy, so he [the Prophet] made him sit behind his back and said: “The mistake of Daawood was looking (i.e. at what is forbidden).” Ibn Al-Qattaan mentioned it in the book, Ahkaam An-Nathar, and he graded it as Dha’eef. Also, Ahmad ibn Is-haaq ibn Ibraaheem ibn Nubayt ibn Shareet reported it in his Nuskhah, and from his route Abu Moosa also narrated it in At-Tarheeb and its chain of narrators is very weak. Shaykh Al-Albaani said in his book, Irwaa’ Al-Ghaleel, about this Hadeeth: “It is fabricated.”