Tafsir of Muhammad 47:19

فاعلم أنه ﻻ إله إﻻ الله

So know that La ilaha illallah, This is a declaration that there is no true God except Allah. It is not something that is easy for him to command the people to have knowledge of. That is why Allah connected it with,

واستغفر لذنبك وللمومنين والمومنات

and ask forgiveness for your sin and for the believing men and women. In the Sahih, it is recorded that Allah's Messenger used to say,

اللهم اغفر لي خطييتي وجهلي وإسرافي في أمري وما أنت أعلم به مني

اللهم اغفر لي هزلي وجدي وخطيي وعمدي وكل ذلك عندي

O Allah, forgive my sin, my ignorance, my immoderation in my affairs, and all that You know better about my (faults) than myself. O Allah, forgive for me my joking, my seriousness, my unintended error, and my deliberate (mistakes) -- and I have done of all that. It is also reported in the Sahih that he used to say at the end of the prayer,

اللهم اغفر لي ما قدمت وما أخرت وما أسررت وما أعلنت وما أسرفت وما أنت أعلم به مني أنت إلهي ﻻ إله إﻻ أنت

O Allah, forgive for me what I have previously done, what I may commit in future, what I have committed secretly, what I have committed openly, what I have done excessively, and all sins that You know better than me. You are my God. There is no (true) God except You. It is also recorded in the Sahih that he used to say,

ياأيها الناس توبوا إلى ربكم فٳني أستغفر الله وأتوب إليه في اليوم أكثر من سبعين مرة

O people! Repent to your Lord! Verily, I ask Allah for forgiveness and repent to Him more than seventy times every day. Allah then says,

والله يعلم متقلبكم ومثواكم

And Allah knows well your moving about, and your place of rest. means, He knows how you behave during the day, and where you settle at night. This is similar to His saying,

وهو الذى يتوفكم باليل ويعلم ما جرحتم بالنهار

It is He Who takes your souls by night and knows all that you do by day. (6:60) And He says,

وما من دآبة في اﻻرض إﻻ على الله رزقها ويعلم مستقرها ومستودعها كل فى كتاب مبين

And there is no moving creature on earth but its provision is up to Allah, and He knows its dwelling place and its deposit (before birth and after death) -- all is in a clear register. (11:6)