מדרש על פסחים 189:7

R. Yehudah says: I might think that "the small Pesach" (Pesach Sheini, viz. Bamidbar 9:10-12) also requires remaining overnight. It is, therefore, written "and you shall betake yourselves in the morning and go to your tents. (8) Six days shall you eat matzoth." What requires six days requires remaining overnight — to exclude the small Pesach, which requires only one day. The sages say: It (Pesach Sheini) is not worse than wood and frankincense. (i.e., Just as they require remaining overnight, so, Pesach Sheini.)