First duty before seeking the knowledge.

Hadis :

لا تدخل الملائكة بيتا فيه كلب

“Malaikat do not enter a house in which there is a dog” 

[Hadith Riwayat al-Bukhari Muslim].

Based on these hadis, Imam Ghazali interpret this hadis :
The meaning behind this hadis based on the first duty before seeking the knowledge.

First we study the meaning behind these thing :
1- Malaikat
Malaikat is created by nur.
In Islam, Knowledge is like nur (light). 

2- House
House = heart

3- Dog
In traditional Islamic literature, dog is as a symbol of desire/will.

What significant with these symbol?

Knowledge never enter the heart which there is a desire there.

Knowledge is like a nur. It will enter the place that clean. The desire that we have in our heart is like the dusk that make our heart dirty. Nur never enter to the dirty place. That why sometime we hard to receive the knowledge because we not purify our heart first before receiving the knowledge.

First duty as a student : Purify the soul