It is narrated from Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) that he went out to the mosque of the Messenger of Allah (peace and blessings be upon him) and saw Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him) sitting and weeping beside the blessed grave of the Messenger of Allah (peace and blessings be upon him). He asked: O Mu'adh! What makes you weep? He said: What makes me weep is something I heard from the occupant of this grave (that is, the Messenger of Allah (peace and blessings be upon him)). He asked: What did you hear? He said: I heard him (peace and blessings be upon him) saying: "Even a little showing off (riya') is shirk (associating partners with Allah). Whoever shows enmity to a friend (wali) of Allah has openly declared war on Allah Almighty. And indeed Allah loves the hidden, God-fearing ones who, if they are absent, are not missed, and if they are present, are not invited (into gatherings) nor recognised. Their hearts are lamps of guidance; they come out (safely) from every dark and dusty trial." This hadith has a sahih (authentic) chain of narration, but the two Shaykhs (Bukhari and Muslim) did not record it.