Masjid is a place to offer 5 daily prayers, as for those places built especifically for prayers alone in work areas or other similar places then those are not masjids, nor do they take the same rule as a majid. Thus, those special rulings such as tahayatul masjid, impermissibilty to sell or buy it, not to publicise lost valubles are all related to masjid alone.

Imam Zarkashi: said ‘the norm is especifically related to masjid which is built to offer 5 daily prayers so that those musalla where people gather to pray Eid or similar prayers are   exempted from its rulings [ref: A’lam Saajid bi Ahkaam al-Masaajid]

As for musalla, it is a place built in deserts or open areas for the purpose to offer  daily prayers, salat ul Eid or salat ul Istisqaa (prayer for rain) and with that other places take similar rule which people have become accustomed to such as a room in hospital or building or at work to offer their prayers.  [ref: mawsu’ah fiqhiya al-kuwaityah 37/195]

Imam Nawawi: said ‘al-musalla is used for salatul Eid and other prayers which is not a masjid, nor its impermissible for a person in state of major impurity to sit in it or for a mensturating woman upon Shaf’ee school of thought and the majority of ulema expressed this view aswell. Imam Daarmee mentioned two opinions with regards to a non-Muslim not permitted to enter musalla except after seeking permission to do so, he mentioned that in chapter of salat ul eid. This view is supported by the hadith of Umm Atiyah that the Prophet of Allah (peace be upon him) ordered the mensturating women to come for Eid prayer but refrain from the musalla but this could be argued that they were commanded not to come near the musalla in order to leave enough space for women who can offer eid prayer and not to overcrowd the area. [ref: Al-Majmoo 2/180]

It is allowed to take musalla in places of work even though the essence is that prayers should be offered in masjids. The evidence for this is the hadith of Ut’baan bin Maalik that he came to messenger of Allah (peace be upon him) and said: ‘O messenger of Allah i have become blind and i offer my prayer with my people. So when it rains, it fills up the valley which is between me and them  preventing me to come to the masjid with them so i pray in my house with them. O messenger of Allah (peace be upon him) i would love you to come to my house and pray there so that i could take that as a musalla. So he said, ‘i will do that inshAllah’. Ut’baan said, ‘the Prophet of Allah and Abu Bakr came (to my house) when the light appeared and he asked my permission to enter so i gave it to him. He did not sit until he entered the house and then said, ‘where would you like me to offer the prayer in your house?’ i then pointed to a particular place and he stood up and made the Takbeer. We also stood up and made rows and then he prayed two rakat and made tasleem.

[ref: agreed upon]

The sunnah establishes a difference between masjid and musalla. A masjid has specific rights which no other place can share and from those are tahayatul masjid. For it has been reported in the hadith of Jabir bin Abdillah who said, ‘a man entered the masjid on day of Jummah whilst the messenger of Allah (peace be upon him) was giving Friday sermon so he said to the man, ‘have you prayed?’ he replied, ‘no’. So he said, ‘then pray two rakat’. [ref: agreed upon]

in another narration from Jaabir; ‘Saleek Al-Ghatfaani came on day of Friday and messenger of Allah was delivering the Friday sermon and he sat. So the messenger of Allah said, ‘if one of you come to Friday prayer whilst the imam is delivering the sermon then you should offer two light rakat and then sit down’ [ref: Muslim]

it has been established from the practice of messenger of Allah (peace be upon him) that he would offer Eid prayer in musalla. So when he would come to the musalla, he would proceed with the prayer without any Adhan or Iqamah so he would offer two rakat consisting several takbeers and he would not offer tahayatul masjid. As it has been reported by Umar bin Shu’aib from his father who reported from his father that Prophet of Allah make 12 takbeers in Eid prayer, 7 in first rakat and 5 in second and he did not offer any other prayer before or after the prayer’ [ref: Ahmed, Ibn Majah & Haafiz Al-Iraqi said that its chain is authentic]

in another narration reported by Abu Sa’eed al-Khudri ‘the Prophet of Allah used to go out on day of Fitr and Adha to the musalla and the first thing he would begin with is the prayer, then he would stand opposite to people and give them admonition’ [ref: agreed upon] this hadith shows that he offered prayer without tahayatul masjid.