﴿ قَالُوا۟ رَبُّنَا يَعْلَمُ إِنَّآ إِلَيْكُمْ لَمُرْسَلُونَ ﴾

“Said [the apostles]: “Our Sustainer knows that we have indeed been sent unto you;”

They said ‘Our Lord knows qālū rabbunā ya‘lamu functions like an oath. Emphasis is intensified by this oath and also by the addition of the lām to what was before simply mursalūna ‘we have been sent’ to counter their intensified denial that we have indeed been sent to you by Him!