Are the sincere ones (al-mukhlaṣīn), mentioned in the Holy Qur’an, exclusively the prophets, peace be upon them, or can this attribute apply to any believer?

The word "al-Mukhlaṣīn" (المُخلَصين) with an open "lām" (fath) is mentioned in the Quran, and sometimes it refers specifically to prophets, and sometimes generally to believers. Regarding His saying: "Except for Your servants among them, al-Mukhlaṣīn" (Al-Hijr: 40), Al-Dahhak said: meaning the believers. And Ibn Jarir said: "Except for those whom You have chosen by Your grace." In His saying: "Except for the servants of Allah, al-Mukhlaṣīn" (As-Saffat: 40), Al-Baghawi said: "The monotheists." It may also refer specifically to prophets, as in His saying about Yusuf: "Indeed, he was one of Our servants, al-Mukhlaṣīn" (Yusuf: 24). Al-Baghawi said: "Meaning those chosen for prophethood." And Al-Qurtubi said: "Those whom Allah has chosen for His message." So the word "al-Mukhlaṣīn" may refer specifically to prophets or generally to believers, and a believer is someone whom Allah has chosen by His grace and guidance.