Berachos, Chapter Two, Mishnah 4. The Mishnah deals with workers who are busy at their labor when the time for Shema arrives:
"Ha'umanin korin b'rosh ha'ilan o b'rosh hanidbach":
The craftsmen, meaning the workers, may recite Shema right where they are standing, even while they are still occupied with their work:
"B'rosh ha'ilan" - while standing at the top of the tree and doing their work.
"B'rosh hanidbach" - at the top of the row of stones of the wall they are building.
Both of these are dangerous places, and the worker has to pay close attention to his task, which makes it hard for him to concentrate on Shema. Even so, since the obligation of concentration in Shema applies only to the first verse, they are required to recite Shema even while standing in these places.
"Mah she'ein rasha'in la'asos kein batefillah":
With Shemoneh Esrei this is not possible. Shemoneh Esrei demands concentration throughout the entire prayer and attention to the meaning of the words, since it is all a plea for mercy before Hashem. It therefore cannot be said in these places, and the worker must come down from where he is in order to pray.
In summary: Shema may be recited at the workplace, at the top of the tree or at the top of the row of stones, because concentration is required only for the first verse; Shemoneh Esrei, however, which is entirely a plea for mercy and requires concentration from beginning to end, is not said there, and one must come down from his place of work in order to say it.