عن أبـي رقــيـة تمـيم بن أوس الـداري رضي الله عنه ، أن النبي صلى الله عـليه وسـلم قـال :" الـديـن النصيحة" قلنا : لمن ؟ قال : " الله ، ولـكـتـابـه ، ولـرسـولـه ، ولأ ئـمـة الـمـسـلـمـيـن وعــامـتهم" رواه مسلم


As for the detailed explanation of Naseehah then al-Khattaabee (rahimhu Allaah) and others from amongst the 'ulemaa have said:
Naseehah to Allaah ta'aalaa refers to belief in Him and the negation of all shirk, and leaving heresy and disbelief in His Attributes, rather describing Him with the Perfect and Complete Attributes, all of them, and freeing Him of all deficiencies. And it further implies being firm upon His obedience and keeping away from sin, and loving for His sake and hating for His sake, and waging Jihaad against those who disbelieve in Him. And recognising His favours and thanking Him for that, and having sincerity for Him in all our affairs, and supplicating to Him with all of His Names and Attributes that He has mentioned, and encouraging others upon that, and courteous behaviour with the people.