Tamim bin Aus Ad-Dari (May Allah be pleased with him) reported:
The Prophet (ﷺ) said, "Ad-Deen is sincerity". We said: "For whom?" He replied, "For Allah, His Book, His Messenger and for the leaders of the Muslims and their masses".[Muslim]. (DE)
Arabic/English book reference (DE): Book 1, Hadith 181 (DE)