I heard we should not kill spiders as a spider constructed a webb to protect Rasulullah (Sallallaahu Alayhi Wasallam) in Ghaar-e-Sawr (cave of Thawr).
Rasulullah (Sallallaahu Alayhi Wasallam) is reported to have said that the Nikah which has the most amount of blessings is the one in which the least amount of expenses are incurred. (Mishkaat pg.268; Qadeemi)
The Ulama have differences of opinion with regards to the killing of spiders. Some Ulama dislike the killing of spiders due to it having made a webb at the cave of Thawr for the protection of Nabi (Sallallaahu Alayhi Wasallam). This is also the view of Hadhrat Ali (Radhiallaahu Anhu). However, Tha?laby and ibn Atiyya have narrated another narration of Hadhrat Ali (Radhiallaahu Anhu), ?Cleanse your homes from the webbs of spiders as leaving it creates poverty.? The second view is supported by those Ahaadith which suggests a person to keep his homes and courtyards clean.? (Ma?ariful Qur’aan part6 pg.682; Darul Kitab)
Al-Mahmood 18