Sayyiduna Abu al-Darda', Sayyiduna Abu Umamah, Sayyiduna Wathilah bin al-Asqa' and Sayyiduna Anas bin Malik (may Allah be pleased with them) narrate that the Messenger of Allah (peace and blessings be upon him) was asked: "Who are those firmly grounded (rasikh) in knowledge?" He said: "He whose oath is true, whose tongue is truthful, and whose stomach and private parts are chaste: that is the one firmly grounded."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7627