For which part? The bit about Lenin’s warning is from Lenin’s Testament[0] in the post-script:

Stalin is too coarse and this defect, although quite tolerable in our midst and in dealing among us Communists, becomes intolerable in a Secretary-General. That is why I suggest that the comrades think about a way of removing Stalin from that post and appointing another man in his stead who in all other respects differs from Comrade Stalin in having only one advantage, namely, that of being more tolerant, more loyal, more polite and more considerate to the comrades, less capricious, etc. This circumstance may appear to be a negligible detail. But I think that from the standpoint of safeguards against a split and from the standpoint of what I wrote above about the relationship between Stalin and Trotsky it is not a [minor] detail, but it is a detail which can assume decisive importance.

Also in the testament he said of Stalin

I think that Stalin’s haste and his infatuation with pure administration, together with his spite against the notorious “nationalist-socialism” [Stalin critised the minority nations for not being “internationalist” because they did want to unite with Russia], played a fatal role here. In politics spite generally plays the basest of roles.

Also there was 21 members and 10 candidate members of the Central Committee at the time of the Revolution in 1917. You can look up the history of each of the members and see that, for the ones who made it through till Lenin’s death, Stalin had them executed in order to consolidate all power. These members include Zinoviev and Kamenev who represented the “left wing” of the party. Trotsky was the only original member that survived but he was banished and never returned to Russia. Stalin would use him as a scapegoat for many years afterwards; using alleged involvement in “Trotskyist groups” as an excuse to imprison or kill other members of the party.

[0] https://www.marxists.org/archive/lenin/works/1922/dec/testamnt/index.htm