‘Amr ibn ‘Absah reported: A man said, “O Messenger of Allah, what is Islam?” The Prophet, peace and blessings be upon him, said:

أَنْ يُسْلِمَ قَلْبُكَ لِلَّهِ عَزَّ وَجَلَّ وَأَنْ يَسْلَمَ الْمُسْلِمُونَ مِنْ لِسَانِكَ وَيَدِكَ

That you surrender your heart to Allah and that Muslims are safe from your tongue and hand.

Source: Musnad Aḥmad 16691, Grade: Sahih