An extract from Lenin's Political Testament commenting on Stalin, as quoted in a UK A Level History textbook:

Comrade Stalin, having become general secretary has immeasurable power concentrated in his hands, and I am not sure that he always knows how to use that power with sufficient control.' (29 December, 1922)

This was later followed by an even more damaging postscript...

'Stalin is too rude, and this fault, entirely acceptable in relations between communists, becomes completely unacceptable in the office of General Secretary. Therefore I propose to the comrades that a way be found to remove Stalin from that post and replace him with someone else who differs from Stalin in all respects, someone more patient, more loyal, more polite, more considerate.' (4 January 1923)

Extracts from the same document as 4A, but quoted in a different textbook. It includes comments from Lenin's testament on all of the leading Bolsheviks.
25th December, 1922

'Comrade Stalin, having become General Secretary, has concentrated an enormous power in his hands; and I am not sure that he always knows how to use that power with sufficient caution. On the other hand, Comrade Trotsky... is distinguished not only by his exceptional ability (he is, to be sure, the most able man in the present Central Committee) but also by his too far-reaching self-confidence and a disposition to be too much attracted by the purely administrative side of affairs.

These two qualities of the two most able leaders of the present Central Committee might, quite innocently, lead to a split; if our party does not take measures to prevent it, a split might arise unexpectedly. I will not characterise the other members of the Central Committee as to their personal qualities. I will only remind you that the October episode of Zinoviev and Kamenev was not, of course, accidental but that it ought as little to be used against them personally as the non-Bolshevism of Trotsky....

Bukharin is not only the. most valuable and biggest theoretician of the party, but may legitimately be considered the favourite of the whole party; but his theoretical views can only with the very greatest doubt be regarded as fully Marxist....

Of course, both these remarks are made by me merely with a view to the present time, or supposing that these two able and loyal workers may not find an occasion to supplement their knowledge and correct their one sidedness'.

Postscript 4th January 1923
'Stalin is too rude, and this fault, entirely supportable in relations amongst us Communists, becomes insupportable in the office of General Secretary. Therefore, I propose to the comrades to find a way to remove Stalin from that position and to appoint to it another man who in all respects differs from Stalin only in superiority-namely, more patient, more loyal, more polite and more attentive to comrades, less capricious.....'