التفسير التبليغي
(They said: "We wish to eat thereof and that our hearts be at rest, and that we may know that you have indeed spoken the truth to us, and that we may be witnesses to it.") (5:113)
And that we may know that you have indeed spoken the truth to us, and that we may be witnesses to it. ("وَنَعْلَمَ أَنْ قَدْ صَدَقْتَنَا وَنَكُونَ عَلَيْهَا مِنَ الشَّاهِدِينَ") "And that we may know" is a conjunction to "we wish to eat" and "that our hearts be at rest." And it would be a proof for us before those among the Children of Israel who did not witness it, so that the believers may increase in firmness of faith, and doubt may be removed from the hearts of the doubters and skeptics, and the disbelievers may believe.
The companions of the table requested reassurance by the descent of the table, and this was out of doubt on their part.
They stated that they would believe after their hearts were at rest: ("and that our hearts be at rest, and that we may know that you have indeed spoken the truth to us").
Source: التفسير التبليغي