Charles Ellicott Commentary 1 Chronicles 15:26

"And it came to pass, when God helped the Levites that bare the ark of the covenant of Jehovah, that they sacrificed seven bullocks and seven rams." — 1 Chronicles 15:26 (ASV)

When God helped the Levites that bare the ark. —Compare 2 Samuel 6:13, And it was so, that when the bearers of the ark of the Lord had gone six paces, he sacrificed oxen and fatlings (singular collective). God had been adverse to those who conducted the Ark on the previous occasion (1 Chronicles 13:9), as was inferred from the sudden death of Uzza.

Now, when the Levites had undertaken the work in due order, and no harm had occurred, it was understood that the Divine goodwill was with the enterprise. That they had borne the holy Ark six paces without any sign of wrath was enough to call forth the grateful offerings of hearts relieved from a dread which only ceased to haunt them when the event proved it to be groundless. Our text, more exact than Samuel, gives the number and kind of the victims then sacrificed.

Others refer the two accounts to different sacrifices, taking Samuel to mean that at every six paces a bullock and a fat sheep were slain by priests stationed all along the course, while they suppose our text to refer to a final sacrifice, offered when the Ark had reached its destination. This solution of the difficulty appears incredible, especially regarding the supposition of priests not mentioned in the narrative.

Another view understands our text in this sense, but makes the offering in Samuel an initial sacrifice of consecration.

But it is not likely that the two sacrifices are really different:

1. because the narrative here is generally parallel with Samuel;
2. and the chronicler may have intentionally paraphrased the older text for the sake of explanation.

(Numbers 23:29 for the sacrifice.)