Assalamu'alaikum warahmatullah, may Allah's blessings be upon you
I have that have caused me a trouble in my mind. Can we as a muslim have fun? If so, then can you explain what does this hadeeth means?
عن معاذ ابن جبل أن رسول الله صلى الله عليه وسلم لما بعث به إلى اليمن قال له إياك والتنعم فإن عباد الله ليسوا بالمتنعمين
Especially with the "At Tana'um" part. Does the true believer doesn't seek any kind of fun activities? (If I remember correctly, this hadith was reported by Imam Ahmad).
Jazakallah Khayr

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, Sallallaahu 'alayhi wa sallam, is His slave and Messenger.

There is no problem that it is not forbidden for a Muslim to enjoy and entertain himself with everything that is not forbidden; this is something definite.

Allah Says (what means): {Say, "Who has forbidden the adornment of Allah which He has produced for His servants and the good [lawful] things of provision?"} [Quran 7:32]

As regards the Hadeeth that you asked us about: it was classified as Saheeh (authentic) by some scholars like Al-Albaani. Al-Ma'lami said: “Its chain of narrators is acceptable.” Other scholars classified it as Dha’eef (weak), such as al-Arna’oot, Muhammad ‘Amr and ‘Abdul-Lateef.

Nonetheless, this was a statement of ‘Umar ibn al-Khattaab as in az-Zuhd authored by Ibn al-Mubarak.

It was also reported that ‘Umar ibn al-Khattaab said: “Beware of enjoyment……and learn to face hardships…” This was reported by Ibn Hibbaan in his Saheeh.

However, these statements (of Companions) do not indicate that it is forbidden to enjoy oneself with what is permissible at all. Rather, it indicates that among the characteristics of perfect and special people among the Slaves of Allah is that they do not exaggerate in enjoyment or in aiming at it all the time.

The guidance for not exaggerating in enjoyment is for a number of reasons:

1) The person engaged in exaggerating in enjoyment can hardly fulfill the rights of Islamic obligations.

2) Exaggeration in enjoyment in eating inherits laziness and negligence; exaggeration in enjoyment in clothes leads to being soft in one’s body, which makes one weak in doing hard work, and it may lead to showing off.

3) The person who is accustomed to exaggerating in enjoyment finds it very difficult to leave it, so he spends his whole time in acquiring it.

4) Exaggeration in enjoyment may lead to being attached to this worldly life and being neglectful about the Hereafter.

Allah knows best.