Shaikhul Islaam Ibn Taymiyyah (rahimahullaah) said:

It is incumbent upon the one who enjoins good and forbids evil that his (act of) enjoining good and forbidding evil is for the sake of Allaah, and that his intention is to rectify the affairs of the one he commands towards good and forbids from evil, and to establish the proofs.  His intention should not be for (the sake of) seeking leadership for himself or for his group, or to diminish the worth of others.”

[Source: Quoted in the treatise titled: Kun Salafiyyan Alal Jaad-dati, page: 62-63, read by Shaikh Saaleh al Fawzaan & introduction by Shaikh ‘Ubaid and Shaikh Ali Naasir al-Faqeehi. (Slightly paraphrased)]