Wikipedia's Yuri Gagarin says:
At about 23,000 feet (7,000 m), Gagarin ejected from the descending capsule as planned and landed using a parachute.
and the section Vostok_programme; Vostok 3KA says:
After one orbit, the descent module successfully re-entered the atmosphere, the mannequin was safely ejected, and the dog and other specimens landed separately in the descent module by parachute.
I assume the ejection of the mannequin was a simulation of an astronaut parachuting rather than remaining in the capsule as it landed.
Earlier Voskhod models used a retractable metal probe, but it was unreliable, especially in windy conditions. So in Soyuz, as the other answers have pointed out, a gamma-ray source is pointed at the ground and the backscatter is measured. The instrument ("Kaktus"), was developed in the early 60s.
That suggests that retropropulsion may have been used during Vostok.
Question: When exactly did soviet capsules begin using retropropulsion immediately before landing? Which spaceflight mission was the first to use it successfully?