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`Abdullah bin `Umar (RA) said:
A man asked the Prophet (ﷺ), "What is the best act of Islam?" He said, "To feed others and to give greetings of Salam (peace) to those whom you know and to those whom you do not know. "
Reference:
Al-Bukhari, cf. Al-Asqalani, Fathul-Bari 1/55, Muslim 1/65.