Hadith 12, 40 Hadith an-Nawawi
On the authority of Abu Hurayrah (may Allah be pleased with him) who said:
The Messenger of Allah (peace and blessings of Allah be upon him) said, “Part of the perfection of one’s Islam is his leaving that which does not concern him.”

A hasan (good) hadeeth which was related by at-Tirmidhi and others in this fashion.
عَنْ أَبِي هُرَيْرَةَ رَضِيَ اللهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صلى الله عليه و سلم "مِنْ حُسْنِ إسْلَامِ الْمَرْءِ تَرْكُهُ مَا لَا يَعْنِيهِ". 
حَدِيثٌ حَسَنٌ، رَوَاهُ التِّرْمِذِيُّ [رقم: 2318] ، ابن ماجه [رقم:3976].