Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:

I do not know of any basis or evidence from the Prophet (blessings and peace of Allah be upon him) to suggest that it is mustahabb to do the marriage contract in the mosque. But if it so happens that the husband and the

guardian of the woman are present in the mosque and they do the marriage contract, there is nothing wrong with that. But as for that being mustahabb in the sense that we would tell them to leave the house and go to the mosque, or make an appointment to meet in the mosque and

do the marriage contract there, this is something for which evidence is required, and I do not know of any evidence to that effect.

Liqa’ al-Baab al-Maftooh, 167/17

Some fuqaha regards it as mustahabb to do the marriage contract in the masjid because of the blessings of the masjid and so that the news would spread faster and widely.

They base it on a da'eef hadith by Tirmidhi.