It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Hold fast to humility, for humility is in the heart. Let no Muslim harm another Muslim, for perhaps a weak person wrapped in worn-out clothes is such that, if he were to swear an oath by Allah, Allah would fulfil it."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7737