The Heights - 7:142
"We made a covenant with Moses for thirty nights and added to it ten nights, so the time appointed by your Lord was completed to forty nights. Moses said to his brother Aaron, "Take my place among my people and act righteously. And do not follow the way of those who do corruption."
Related:
The Cow/2:51
We had contracted with Moses for forty nights. But you wronged yourselves after him and served the calf.