Jamieson, Fausset and Brown on 1 Chronicles 15:26

26. it came to pass, &c.--(See on 2Sa 6:13-23).

they offered seven bullocks and seven rams--The Levites seem to have entered on this duty with fear and trembling; and finding that they might advance without any such indications of divine wrath as Uzza had experienced (1Ch 13:10), they offered an ox and a fatted sheep immediately after starting (2Sa 6:13), and seven bullocks and seven rams--a perfect sacrifice, at the close of the procession (1Ch 16:1). It is probable that preparations had been made for the offering of similar sacrifices at regular intervals along the way.