Quite a few versions of the riddle are available, but most of these probably represent some distortion of the form in which it was familiar to Sophocles' audience. Probably the most familiar today runs something like this, available on the web at History For Kids: "What goes on four feet in the morning, two feet at noon, and three feet in the evening?"
Ancient Greek sources, such as Apollodorus and Athenaeus, on the other hand, give a different emphasis to the riddle.
Apollodorus' version is the more widely available. Among other places, it can be found at the Perseus website. It runs as follows: "What is that which has one voice and yet becomes four-footed and two-footed and three-footed?"
Finally, Athenaeus' somewhat fuller version, available at a Sophocles website, runs as follows:
"A thing there is whose voice is one;
Whose feet are four and two and three.
So mutable a thing is none
That moves in earth or sky or sea.
When on most feet this thing doth go,
Its strength is weakest and its pace most slow."