Sahih Bukhari#4987 – Collection Of Quran During Era Of Hazrat Uthman (rta)
This has been discussed at length in Response to 23 Questions Series on the topic of Variant Readings of Quran. If you cannot watch the whole series then following episodes are relevant to your question where the above narration is also discussed. Watch Parts 97, 98 and 99 which are available in the following thread : Discussion 71280