Fronds of the Date palm tree
Sa’f (plural; Sa’afah singular): Sa’f are the branches of the palm tree, consisting of the branch and fronds. sa’f refers to when it is dried. It is called Shatbah while still green. Sa’f also refers to the fronds, while barren branches are called Jarid and the thorns are called Silla’a.
The word Sa’f hasn’t been mentioned in the Holy Qur’an.
Anas b. Malik reported that the Messenger of Allah (may peace be upon him) wits the best among people in character. On occasions, the time of prayer would come while he was in our house. He would then order to spread the mat lying under him. That was dusted and then water was sprinkled over it. The Messenger of Allah (may peace be upon him) then led the prayer and we stood behind him, and that mat was made of the leaves of date-palm. Sahaih Muslim, book of the Prayers