The Muslims were experiencing great hunger so they used to put a rock in a cloth and tie it around their abdomen so that the rock would press against their stomach and give it a sense of fullness. It was customary in those days to tie rocks to one’s stomach in order to alleviate the pain resulting from prolonged hunger.
Hadith mentioned is narrated in Sahih el-Bukhari. Abu Hurayrah is reported to have said:
"Once I was so hungry that I tied a stone to my stomach to suppress the feeling of hunger. "
Abu Hurayra said, "By Allah, there is no god but He. I used to lie with my stomach on the ground out of hunger and I used to tie a stone on my belly because of hunger."
Jabir said, "On the Day of the Ditch, we were digging and a hard bit of rock became exposed. They went to the Prophet, may Allah bless him and grant him peace, and said, "'There is a hard bit of rock exposed in the Ditch.' He said, 'I am coming down.' He got up and there was a stone tied to his belly. We had been three days without eating at all.
In one variant Anas said, "I came to the Messenger of Allah, may Allah bless him and grant him peace, one day and I found him sitting with his Companions and he had tied a band around his stomach. I asked some of his Companions, 'Why has the Messenger of Allah bound up his stomach.' They said, 'Out of hunger.' So I went to Abu Talha, who was the husband of Umm Sulaym bint Milhan, and said, 'O father, I have seen the Messenger of Allah, may Allah bless him and grant him peace, with his stomach tied with a band and I asked some of his Companions and they said, "It is from hunger."
Pretty kewl huh?!