I am looking for a narration full version and reference. In the narration, the Prophet SAWS and sahabas were sitting. A sahaba said he loves another sahaba. The Prophet instructed him to go and tell the other man he loves him.
A man was with the Prophet (ﷺ) when another man passed by and the former said: "O Messenger of Allah! I love this man (for Allah's sake)". Messenger of Allah (ﷺ) asked, "Have you informed him?" He said, "No". Messenger of Allah (ﷺ) then said, "Tell him (that you love him)". So he went up to the man and said to him, "I love you for the sake of Allah;" and the other replied, "May Allah, for Whose sake you love me, love you." [Riyad as-Salihin 385]
The hadith appears in Sunan abi Dawod (where you my find a special subsection entitled:"Chapter: When one man loves another because of some good that he sees"):
Narrated Anas ibn Malik:
A man was with the Prophet (ﷺ) and a man passed by him and said: Messenger of Allah! I love this man. The Messenger of Allah (ﷺ) then asked: Have you informed him? He replied: No. He said: Inform him. He then went to him and said: I love you for Allah's sake. He replied: May He for Whose sake you love me love you!
Imam an-Nawawi qualified its narrator chain as "sahih" and al-Albani qualified the hadith as hassan.
In the same section you may find a more general hadith saying:
Narrated Al-Miqdam ibn Ma'dikarib:
The Prophet (ﷺ) said: When a man loves his brother, he should tell him that he loves him.
(Sunan abi Dawod and see also in al-Adab al-Mufrad)
This hadith is qualified as sahih in Sunan abi Dawod and as hassan sahih in imam al-Bukhari's longer version in al-Adab al-Mufrad by al-Albani.