In The Name of Allaah, The Most Merciful, The Bestower of Mercy.

Sincerity:

Jubayr Ibn Mut’im [radiyallaahu-anhu] said: The Prophet [sallal-laahu-alayhi-wasallam] said: ”There are three things the heart of the believer does not harbour hatred towards: Performance of deeds sincerely for the sake of Allaah, giving sincere advise to those in charge of the affairs of the Muslims [i.e. the Muslim rulers] and adhering to the Jamaa’ah [i.e. adhering to the Scholars upon the truth and not distancing oneself from unity behind a Muslim ruler, and not rebelling against the ruler]. For indeed, their supplication surrounds those behind them [i.e. the ones under their authority]. [Reported by Ibn Maajah (Number:3056-also see saheeh At-targheeb Wat-Tarheeb (No:92) of Imaam albaanee (rahimahullaah)]

And what is well known is that the one who is sincere in his religion will not carry (in his heart), except sincere love for his Muslim brothers. And with this he becomes happy when good (reaches) them and he feels sad when calamity befalls them, whether that (occurs) in their religious or worldly affairs.

The Slave Satisfies Himself With His Lord

Imaam Ibnul Qayyim (rahimahullaah) said: ”Being satisfied and sufficed (with Allaah) opens a door of safety for the slave, and it cleanses his heart of deception, corruption and rancour. And none will be saved from the punishment of Allaah, except the one who comes to Allaah (on the Day of Judgement) with a pure heart. Likewise, there cannot be peacefulness of the heart alongside discontentment and an absence of satisfaction (with Allaah); and whenever the slave’s satisfaction (with his Lord) becomes stronger, his heart is more at peace. So malice, corruption and deception are companions of discontentment, and peacefulness, piety and sincerity of the heart are companions of satisfaction. Likewise, envy is from the fruits of discontentment and the heart’s safety from it is from the fruits of satisfaction.”  [Madaarij As-Saalikeen: 2/207]

Supplication: 

The slave should always supplicate to Allaah to make his heart settle upon peacefulness towards his brothers. And he should supplicate for them, because this is a persistent habit of the righteous people.  Allaah (The Most High) said: ”And those who came after them say: “Our Lord! Forgive us and our brethren who have preceded us in Faith, and put not in our hearts any hatred against those who have believed. Our Lord! You are indeed full of kindness, Most Merciful.” [Soorah Al-Hashar: Ayah:10] [Source: Inshiraah As-sadr:page:162-164]