قَالُوا۟ يَٰٓأَبَانَآ إِنَّا ذَهَبْنَا نَسْتَبِقُ وَتَرَكْنَا يُوسُفَ عِندَ مَتَٰعِنَا فَأَكَلَهُ ٱلذِّئْبُ وَمَآ أَنتَ بِمُؤْمِنٍ لَّنَا وَلَوْ كُنَّا صَٰدِقِينَ
"They said: 'Our father! We went off racing, and we left Joseph by our belongings, and the wolf ate him. But you will not believe us, even though we speak the truth.'"
Root س-ب-ق, Form VIII: to race. When the verb was analysed at al-Nāziʿāt 79/1-5 (fa's-sābiqāti sabqā), this very verse was cited:
سَابَقَ — he raced. "They raced" (at Yūsuf 12/17 nastabiqu — we were racing).
And the same verb will occur once more in the sūra: wa'stabaqa'l-bāb (25) — "they raced for the door." In both scenes there is a race; in one a losing is told, in the other an escape. This is a repetition verifiable within the sūra.
The work of the sentence must be recorded, and I give this as my own reading: immediately after the news is delivered, a sentence about the credibility of the news is added. That is, the speaking party voices the listener's doubt itself.
And this is a common behavioural pattern of lying: naming the doubt in advance makes the doubt look unjust. By adding the clause "even though we speak the truth" the sentence draws the listener onto the defensive.
I give this as a reading; the verse does not make this connection explicitly, but the place of the sentence in the verse (immediately after the news) lends itself to it.
بِمُؤْمِنٍ لَّنَا — note: āmana bi- means "to believe in something", while āmana li- means "to credit someone, to believe his word". The li here shows that the object of the belief is a report.