Tafsir of Ta-Ha 20:83

Musa goes to the Appointment with Allah and the Children of Israel succumb to worship the Calf Allah relates what happened when Musa traveled with the Children of Israel after Fir`awn's destruction

فٲتوا على قوم يعكفون على أصنام لهم قالوا يموسى اجعل لنا إلها كما لهم ءالهة قال إنكم قوم تجهلون

إن هوﻻء متبر ما هم فيه وبطل ما كانوا يعملون

And they came upon a people devoted to some of idols. They said: "O Musa! Make for us god as they have gods.'' He said: "Verily, you are a people who know not. Verily, these people will be destroyed for that which they are engaged in. And all that they are doing is in vain.'' (7:138-139) Then, Allah made a covenant with Musa of thirty nights after which He added to them ten more nights. Thus, they were forty nights in all. The covenant was that he was to fast these number of days, during both the day and night. Thus, Musa made haste to go to the Mountain and he left his brother, Harun, in charge over the Children of Israel. This is why Allah says,

وما أعجلك عن قومك يا موسى