Sufi wandering rather than organized missions spread Islam eastward

Evidence

Muslims did not engage in organized missionary activity. But of course there were the wandering Sufis who carried the faith, particularly eastwards into Cen­ tral Asia, India and other places. We have much less record of that, though there is some. We have accounts also by disciples, both of holy men and of learned teachers, who traveled great distances to sit at the feet of the great masters of the faith. Some of them even compiled books containing short biographies of the teachers whose courses they had attended. — Bernard Lewis, From Babel to Dragomans, 739