Musnad Ahmad 1651

It was narrated that "Abdullah bin "Abdur-Rahman bin Abi Husain said: I heard that Luqman used to say: O my son, do not acquire knowledge in order to boast about what you have of knowledge when you are with scholars or to use it in argument with the foolish and to show off in gatherings. And he said: Nawfal bin Musahiq told us, from Sa"eed bin Zaid, that the Prophet (ﷺ) said: "One of the worst types of riba is to slander the honour of a Muslim with no justification. And this rahim (ties of kinship) is derived from ar-Rahman; whoever breaks ties of kinship, Allah will forbid Paradise to him.”