Is it permissible to perform اعتکاف (spiritual retreat) in the Prophet's Mosque and reserve a spot in the first row, then go to sleep at the back of the mosque and return to the reserved spot, and is it permissible to sleep in the first row outside of prayer times?

It is permissible for someone in the mosque to place a prayer rug or similar item in their spot and then sleep at the back of the mosque, returning to their spot as long as the prayer has not yet begun. They have the right to their spot if they left for an excuse, such as performing ablution, and then returned. However, if they were negligent in returning, they do not have a right to it.

The proof for this is the saying of the Prophet, peace and blessings be upon him: "Whoever gets up from his sitting place and then returns to it, he has the most right to it."

If someone reserves a spot and leaves the mosque due to a sudden need then returns shortly after, they have the most right to it. But if they delay their return after their excuse has ended, they do not have the most right to it. There is no harm for someone performing I'tikaf (spiritual retreat) or others in sleeping in the first row between prayers, as long as it does not cause harm or constriction to anyone. Nevertheless, it is preferable to avoid sleeping in the front rows out of consideration for others.