What is the virtue of declaring Tawhid at the time of death?

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Whosoever's last statement at death is 'I testify that there is no god but Allah, alone, without partner,' it will erase all previous sins and transgressions. This is narrated regarding Mu'adh ibn Jabal. Furthermore, this declaration is even more effacing for the living, as stated by the Prophet (peace be upon him) when asked about its effect on the living.