What makes this difficult is the way Bitcoin can be used. Bitcoin can both be a commodity and a currency and this is where the answer to this question gets complicated. According to Muslim religion you are not allowed to "loan" at interest to someone. <right?> and you are not allowed to gamble.
Some might see a Bitcoin investment as "gambling" ...because you are hoping that the price might rise.. and this is gambling? If you just use it to pay someone, no gambling is involved.