The Secret Behind the Connection of لا إله إلا الله

The secret behind the connection of

لا إله إلا الله 

and seeking forgiveness in the saying of the Most High, 

فاعلم أنه لا إله إلا الله و استغفر لذنبك و للمؤمنين والمؤمنات 

"And have knowledge that none has the right to be worshipped except Allaah and seek forgiveness for your sins and for the believing men and women". 

Tawheed removes the roots of shirk and forgiveness removes it's branches. 

The most complete form of praise is  saying "لا إله إلا الله" And the most complete form of supplication is Istighfaar (seeking forgiveness). 

So Allaah has ordered one with Tawheed and Istighfaar for him and for his brothers from the believers (and sisters) from the believing women.

Ibn Taymiyyah 11/697 Majmou Fataawa