What he told the young men after twenty nights
He worked out that they were homesick before they said so.
ارْجِعُوا إِلَى أَهْلِيكُمْ فَعَلِّمُوهُمْ وَمُرُوهُمْ، وَصَلُّوا كَمَا رَأَيْتُمُونِي أُصَلِّي
Irjiʿū ilā ahlīkum fa-ʿallimūhum wa murūhum, wa ṣallū kamā raʾaytumūnī uṣallī
“Go back to your families, and teach them and instruct them, and pray as you have seen me pray.”
Where this comes from
They were young men of about the same age who had stayed with him twenty nights. Malik ibn al-Huwayrith says he guessed they were missing their people, asked who they had left behind, and he was gentle and merciful.
The full narration
Narrated Abu Sulaiman and Malik bin Huwairith:We came to the Prophet (ﷺ) and we were (a few) young men of approximately equal age and stayed with him for twenty nights. Then he thought that we were anxious for our families, and he asked us whom we had left behind to look after our families, and we told him. He was kindhearted and merciful, so he said, "Return to your families and teach them (religious knowledge) and order them (to do good deeds) and offer your prayers in the way you saw me offering my prayers, and when the stated time for the prayer becomes due, then one of you should pronounce its call (i.e. the Adhan), and the eldest of you should lead you in prayer
Source
Ṣaḥīḥ al-Bukhārī · 6008
- Agreed upon as authentic (al-Bukhārī / Muslim)