Joking and talking inside the mosque before praying, is it forbidden?

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 is His slave and Messenger.

Ibn ‘Umar narrated that the Prophet said: “Indeed, I make jokes, but I only say the truth.” [Al-Tabaraani]

His joke was a rarity and it was used to make his companions happy. The companions of the Prophet used to make jokes. But they did not use to make much jest. Actually, whoever makes many jokes, this will have a bad effect on him and will decrease his solemnity. It can even make him worthless in the eyes of people.

It is reported from ‘Abdul-‘Azeez Ibn Daawood that ‘Umar ibn ‘Abdul-‘Azeez said to some of his companions: “Be pious and do not fear but Allah Who has no associates. Beware of joking as it leads to bad things and leads to rancor (between people). Get together on the Quran and talk about it. If this becomes a burden on you, then tell some good stories.”

So, limit your jokes and recite the Quran as much as possible and remember Allah. You can also read some good books.

Know also that jokes should be only with people who can stand them; if you feel the opposite, you must stop these jokes as harming Muslims is a sin. Allah Says (what means): {And those who harm believing men and believing women for [something] other than what they have earned have certainly born upon themselves a slander and manifest sin.} [Quran 33: 58]

Any person who enters the mosque should occupy himself with the prayer, Du'aa’ (supplication), or remembrance of Allah. But it is lawful to talk about matters of this worldly life as well as the Hereafter as long as the person does not talk loudly. But there are some acts that are forbidden in the mosque such as buying, selling, asking about a lost thing, … etc.

An-Nawawi said: “It is lawful to say lawful things and to talk about lawful matters in the mosque even if someone laughs as longs as what is said is lawful. This is from the Hadeeth of Jaabir ibn Samurah that he said: “He (the Prophet) used to sit at the place where he observed the Fajr prayer till the sun rose or when it had risen; he would stand; the Companions would talk about matters pertaining to Jaahiliyyah (the pre-Islamic period of ignorance), and they would laugh while the Prophet would only smile." [Muslim]

Allah knows best.