Musnad Ahmad 9:27

Abu al-Yaman narrated to us, Shu’ayb informed us, from ‘Abdullah ibn ‘Abd al-Rahman ibn Abi Husayn, who said: It reached me that Luqman used to say: "O my son, do not learn knowledge to boast before the scholars, or to argue with the foolish, or to show off in gatherings." Then he mentioned it. And he said: Nawfal ibn Musahiq narrated to us, from Sa’id ibn Zayd, from the Prophet, peace and blessings of Allah be upon him, that he said: "Among the most grievous of usury is the excessive prolongation in [attacking] the honor of a Muslim without right. And indeed, this kinship (rahim) is a branch from the Most Merciful (al-Rahman); so whoever severs it, Allah has forbidden Paradise to him."