"Law ma," meaning: why do you not "bring us the angels" so that they may testify that you are the Messenger of Allah "if you are among the truthful," so that we may believe you.
﴿لَو مَا﴾ أَي: لَوْلَا ﴿تَأْتِينَا بِالْمَلَائِكَةِ﴾ حَتَّى تَشْهَدَ أَنَّكَ رَسُولُ اللَّهِ ﴿إِن كنت من الصَّادِقين﴾ فنصدقك.
