How to Respond Upon Hearing the Athan (Call to Prayer)?

The Sunnah recommends attentively listening to the Athan (call to prayer) when it is being called by the Mu'adhdhin (the one who calls to prayer).
Every line should be silently repeated after the Mu'adhdhin except for when he says حَيَّ عَلَىٰ ٱلصَّلَوٰةِ (come to prayer) and حَيَّ عَلَىٰ الۡفَلَاحِ (come to salvation). At these two points, one should say:
لَاحَوۡلَ وَلَا قُوَّةَ إِلَّا بِٱللَّهِ (There is no might and no power except with Allah)

Let’s see what one such Hadith says about responding to the Athan, followed by the reward for Muslims performing this deed:

Umar bin Al-Khattab reported:
"The Prophet (SAW) said:

When the Mu'adhdhin says:
ٱللَّهُ أَكۡبَرُ ٱللَّهُ أَكۡبَرُ(Allah is the Greatest, Allah is the Greatest)
And one of you should make this response:
ٱللَّهُ أَكۡبَرُ ٱللَّهُ أَكۡبَرُ(Allah is the Greatest, Allah is the Greatest)

(and when the Mu'adhdhin) says:
أَشۡهَدُ أَنۡ لَآ إِلَٰهَ إِلَّا ٱللَّهُ(I testify that there is no god but Allah)
One should respond:
أَشۡهَدُ أَنۡ لَآ إِلَٰهَ إِلَّا ٱللَّهُ(I testify that there is no god but Allah)

And when he (Mu'adhdhin) says:
أَشۡهَدُ أَنَّ مُحَمَّدٗ رَّسُولُ ٱللَّهِ(I testify that Muhammad is the Messenger of Allah)
One should make a response:
أَشۡهَدُ أَنَّ مُحَمَّدٗ رَّسُولُ ٱللَّهِ(I testify that Muhammad is the Messenger of Allah)

When he (the Mu'adhdhin) says:
حَيَّ عَلَىٰ ٱلصَّلَوٰةِ(Come to prayer)
One should make a response:
لَاحَوۡلَ وَلَا قُوَّةَ إِلَّا بِٱللَّهِ(There is no might and no power except with Allah)

When he (the Mu'adhdhin) says:
حَيَّ عَلَىٰ الۡفَلَاحِ(Come to salvation)
One should respond:
لَاحَوۡلَ وَلَا قُوَّةَ إِلَّا بِٱللَّهِ(There is no might and no power except with Allah)

And when he (the Mu'adhdhin) says:
ٱللَّهُ أَكۡبَرُ ٱللَّهُ أَكۡبَرُ(Allah is the Greatest, Allah is the Greatest)
Then make a response:
ٱللَّهُ أَكۡبَرُ ٱللَّهُ أَكۡبَرُ(Allah is the Greatest, Allah is the Greatest)

When he (the Mu'adhdhin) says:
لَآ إِلَٰهَ إِلَّا ٱللَّهُ(There is no god but Allah)
And he who makes a response from the heart:
لَآ إِلَٰهَ إِلَّا ٱللَّهُ(There is no god but Allah),

He will enter paradise."

– Reference: Sahih Muslim 385

An act as simple as answering the call to prayer can be the reason for a believer entering Jannah.
Indeed, Allah Subhanahu wa Ta’ala is the Most Merciful. He allows His servants numerous opportunities to be showered with His many blessings and form a strong connection with the Almighty.