Why was charity specifically mentioned as the first deed in Allah's Almighty saying: "And spend [in the way of Allah ] from what We have provided you before death comes to one of you and he says, 'My Lord, if only You would delay me for a brief term so I would give charity and be among the righteous'"?

The charity mentioned in the verse is the obligatory Zakat (definition), and Allah has commanded its prompt payment before death, because whoever has not paid it will wish to return to pay it after it is too late. The verse indicates the obligation to hasten the performance of Zakat and all other acts of worship when their time is due. Ibn Abbas interprets the verse as referring to whoever was obligated to pay Zakat or perform Hajj but did not do so, and then requests to return [to life] at the time of death.