Specifically, FAR 25.111 states that:
The airplane must reach V 2before it is 35 feet above the takeoff surface and must continue at a speed as close as practical to, but not less than V 2, until it is 400 feet above the takeoff surface;
and
The airplane configuration may not be changed, except for gear retraction and automatic propeller feathering, and no change in power or thrust that requires action by the pilot may be made until the airplane is 400 feet above the takeoff surface;
The 400 foot idea seems to have been introduced in SR 422, which was the 1957 change to certification in anticipation of turbine powered transports. I don't see it referred to in CAR 4b as it was in 1953.
This gives me the chance to promulgate my pet theory about the change from a 50 foot screen height to a 35 foot screen height. I have never been able to find a true reason for this, but consider that, a) Boeing had bet more than the net worth of the company on the 707 project, b) the 707 in its original form couldn't make a 50 foot screen when operating from many of the existing runways in the U.S. (that had been designed for piston transports), and c) Boeing was the prime contractor for some very significant defense projects at the time (B-52, Bomarc, etc.)
At the risk of opening the Pandora's box of government subsidization and the like, I have often wondered whether it simply came down to making sure the 707 worked so that Boeing stayed solvent in the middle of the Cold War. I have absolutely no grounds for this theory, but it used to be a fun story to use when teaching performance to turboprop (50 foot screen height) pilots back in the eighties.