Prophet Nuh ’alayhis salaam said to his people:
Ask forgiveness of your Lord. Indeed, He is ever a Perpetual Forgiver. He will send [rain from] the sky upon you in [continuing] showers. And give you increase in wealth and children and provide for you gardens and provide for you rivers.
[Nuh: 10-12]
Prophet Hud ’alayhis salaam said to his nation:
And O my people, ask forgiveness of your Lord and then repent to Him. He will send [rain from] the sky upon you in showers and increase you in strength [added] to your strength. And do not turn away, [being] criminals.
[Hud: 52]
The Prophet sallallahu ’alayhi wa sallam said,
Whoever persists in asking forgiveness, Allah will make for him, for every grief, a way out, for every difficult situation, an exit, and He will provide him with sustenance from places that he did not expect.
[Abu Da'ood & Musnad Ahmad]
The proper way of repenting that is praised in the Qur’an and Sunnah is not just merely saying “Astaghfirullah”. There are certain conditions that need to be met for a truly and sincere repentance:
Your intention for repentance must be purely for the sake of Allah
You must immediately stop committing the sin
You must make the intention that you will not return to committing the sin
You should feel guilty over committing the sin in the first place
You should seek Allah’s forgiveness both verbally (asking for Allah’s Forgiveness, Mercy, and Blessings) and through actions (performing good deeds)
If the sin involved harming someone, their forgiveness should be sought as well
