Musannaf Ibn Abi Shayba 33:9

Jarir ibn Abd al-Hamid narrated to us, from Abd al-Aziz ibn Rufay’, from Abu Thumamah, who said: The disciples said, "O Isa, what is sincerity (ikhlas) for Allah?" He said, "That a man performs a deed and does not love that anyone among the people should praise him for it." And the one who is sincere (munasih) to Allah is the one who begins with the right of Allah before the right of the people, and prefers the right of Allah over the right of the people. And if two matters are presented to him, one of them for the worldly life and the other for the Hereafter, he begins with the matter of the Hereafter before the matter of the worldly life.