Al-Barā' ibn ‘Āzib (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "When a Muslim is questioned in the grave, he testifies that There is no god but Allah and that Muhammad is the Messenger of Allah. This is the explanation of the words of Allah the Exalted: ﴿Allah will keep firm those who believe, with the firm statement in this world and the Hereafter.﴾ [Sūrat Ibrahīm: 27]"
شرح الحديث :
The believer will be questioned in the grave by two angels assigned for this purpose, whose names are Munkar and Nakīr, and he will testify that There is no god but Allah and that Muhammad is the Messenger of Allah. This is the "firm statement" that Allah mentions in the Qur'an: ﴿Allah will keep firm those who believe, with the firm statement in this world and the Hereafter.﴾ [Sūrat Ibrahīm: 27]