لِأَسْبُرَ مَجْلَبَ الدَّمْعِ
In order to discover what was causing the tears.
Asbura comes from sabara, to probe or examine deeply. Majlabat al-damʿ means what brings tears about.
The purpose particle li- explains why he entered the crowd: not simply to watch, but to discover the cause of the weeping.