Why prostration in prayer must be done on earth  Abu Said Al
Khudri narrates:  The Messenger of Allah used to practice Itikaf
(in the mosque), in the middle third of Ramadan. After the
twentieth night, he used to return home on the twenty-first, and
those who were in Itikaf with him would return to their homes as
well. In Ramadan, in which he practiced Itikaf, he would pray
the night prayers on the night in which he returned home, and
then address the people, instructing them as Allah commanded
him. He said, "I used to practice Itikaf for these ten days
(i.e. the middle third of Ramadan), but I now intend to stay in
Itikaf for the last ten days. Whoever was in Itikaf with me
should stay at his place of seclusion. I have certainly been
shown (the date of) this Night (of Qadr), but I have forgotten
it. Search for it in the odd nights of the last ten days (of
Ramadan). I also saw myself (in a dream) prostrating in mud and
water." On the twenty-first night, the sky was covered with
clouds and it rained, and the r