Stalin was one of Lenin's most trusted comrades, and was often Lenin's first pick for dispatching to critical hotspots during the civil war. Even post-war, he remained a close confidant.

The log of Lenin’s activities during this time (May 25-Oct. 2, 1922) indicates Stalin to have been the most frequent visitor to Gorky, meeting with Lenin 12 times; according to Bukharin, Stalin was the only member of the Central Committee whom Lenin asked to see during the most serious stages of his illness. According to Maria Ulianova, these were very affectionate encounters: “Lenin met [Stalin] in a friendly manner, he joked, laughed, asked that I entertain him, offer him wine, and so on. During this and further visits, they also discussed Trotsky in my presence, and it was apparent that here Lenin sided with Stalin against Trotsky.” Lenin also frequently communicated with Stalin in writing. His archive contains many notes to Stalin requesting his advice on every conceivable issue, including questions of foreign policy. Worried lest Stalin overwork himself, he asked that the Politburo instruct him to take two days’ rest in the country every week. After learning from Lunacharsky that Stalin lived in shabby quarters, he saw to it that something better was found for him. There is no record of similar intimacy between Lenin and any other member of the Politburo. After obtaining Lenin’s consent and then settling matters among themselves, the triumvirate [Stalin, Zinoviev, Kamenev] would present to the Politburo and the Sovnarkom resolutions that these bodies approved as a matter of course. Trotsky either voted with the majority or abstained. By virtue of their collaboration in a Politburo that at the time had only seven members (in addition to them and the absent Lenin, Trotsky, Tomsky, and Bukharin), the troika could have its way on all issues and isolate Trotsky, who had not a single supporter in that body.

Pipes, Richard. Russia Under the Bolshevik Regime. New York: A.A. Knopf, 1993, p. 464-466

Until the end of 1922, Stalin’s relations with Lenin were extremely close. From the end of May until the beginning of October in that year, Stalin visited Lenin at Gorky 12 times, more often than any other person. As Lenin’s sister Maria wrote to the Presidium of the Combined Plenum of the Central Committee and Central Control Commission of July 1926: “Lenin valued Stalin very highly…. Lenin used to call him out and would give him the most intimate instructions, instructions of the sort one can only give to someone one particularly trusts, someone one knows as a sincere revolutionary, as a close comrade…. In fact, during the entire time of his illness, as long as he had the possibility of seeing his comrades, he most frequently invited Comrade Stalin, and during the most difficult moments of his illness Stalin was the only member of the Central Committee he invited.” This letter was written to bolster Stalin in the savage internecine struggle going on in the leadership, but it nevertheless reflects the reality.

Volkogonov, Dmitrii. Lenin: A New Biography. New York: Free Press, 1994, p. 268