What is the first thing that the Prophet (blessings and peace of Allah be upon him) instructed the people to do when he first came to Madinah?

Praise be to Allah.

The first thing that the Prophet (blessings and peace of Allah be upon him) is known to have spoken about and instructed the people to do when he first arrived in Madinah was when he said: “O people, spread (the greeting of) salaam, offer food to people and pray at night when people are sleeping; you will enter Paradise in peace.”

It was narrated by at-Tirmidhi (2485), who classed the report as sahih, and by Ibn Majah (1334) and Ahmad (23784) that ‘Abdullah ibn Salaam (may Allah be pleased with him) said: When the Messenger of Allah (blessings and peace of Allah be upon him) came to Madinah, the people rushed towards him and it was said, The Messenger of Allah (blessings and peace of Allah be upon him) has come! I came along with the people to see him, and when I looked at the face of the Messenger of Allah (blessings and peace of Allah be upon him), I realized that his face was not the face of a liar. The first thing he said was, “O people, spread (the greeting of) salaam, offer food to people and pray at night when people are sleeping; you will enter Paradise in peace.” Al-Albani classed it as sahih in Sahih Sunan at-Tirmidhi.

And Allah knows best.