Smith achieved the feat during the second innings of the first Ashes Test match against England. Smith recorded his 25th century in 119th innings and went past Indian skipper Virat Kohli who had achieved the feat in 127 innings.

from Times of India https://timesofindia.indiatimes.com/sports/cricket/ashes-2019/ashes-2019-steve-smith-becomes-second-fastest-batsman-to-score-25-test-centuries/articleshow/70523406.cms