a<-matrix(c(1,2,3,4,5), nrow = 10, ncol = 1)
write.csv(a,row.names=FALSE, col.names=FALSE, file = "D:/a/matrix.csv")

Warning message:
In write.csv(a, row.names = FALSE, col.names = FALSE, file = "D:/a/matrix.csv") :
  attempt to set 'col.names' ignored