incumbent (up)on (someone) to (do something)
Imposed or expected as an obligation, duty, or requirement on someone. As the wealthier citizens of the country, it's my firm belief that it is incumbent upon us to contribute toward the welfare of those who are less fortunate. He felt it was incumbent on him to take over the responsibilities of his father's business.
incumbent (upon someone) to do (something)
obligatory for someone to do something. It is incumbent upon me to inform you that you are up for review. It was incumbent on Mary to mail her application before June 1st.