And they were not commanded but to serve Allah, being sincere to Him in obedience, and being upright, and to observe Prayer, and pay the Zakat. And that is the religion of the people of the right path.
ISLAM, THE HOLY QUR’AN, 98:6
O people, make your deeds sincere for Allah Almighty. Verily, Allah does not accept any deed unless it is sincerely for Him. Do not say: This is for the sake of Allah, and this is for the sake of my relatives. Verily, it was done for your relatives and none of it was for Allah. Do not say: This is for the sake of Allah and for your sake. Verily, it was done for their sake and none of it was for Allah.
ISLAM, THE HOLY PROPHET (SA), SUNAN AL-DARAQUTNI
God reveals Himself only to those who search for Him with love and sincerity of heart. He manifests Himself unto those who become His alone. Pure hearts are the throne upon which He settles. Tongues which are untainted by falsehood, abuse, and absurdities are the abode of His revelation. And those who immerse themselves in His pleasure become a manifestation of His miraculous power.
ISLAM, THE PROMISED MESSIAH (AS), THE TRUTH UNVEILED, P. 15
You shall love the Lord your God with all your heart, with all your soul, and with all your might.
JUDAISM & CHRISTIANITY, DEUTERONOMY 6.5
It matters not whether you do much or little, so long as your heart is directed to Heaven.
JUDAISM, TALMUD, BERAKOT 17A
Sincerity is the single virtue that binds divinity and man in one.
Like a clear mirror
Reflecting images according to the forms,
So from Buddha’s field of blessings
Rewards are obtained according to one’s heart.
BUDDHISM, GARLAND SUTRA 10
A man becomes pure through sincerity of intellect; thereupon, in meditation, he beholds Him who is without parts.
HINDUISM, MUNDAKA UPNISHAD 3.1.8