This sentence occurs in LLPSI (Orberg) Cap XLI:
The robbers have struck back at Romulus and Remus who have been stealing from them.
Sed latrones ob praedam amissam irati geminis insidias fecerunt et, cum Romulus vi se defendisset, Remum ceperunt et captum regi Amulio tradiderunt falso accusantes 'in Numitoris agros ab iis impetus fieri; inde eos, collecta iuvenum manu, hostilem in modum praedas agere!'
But the thieves, angered by losing their booty, made an ambush against the twins and, whilst Romulus, with force, defended himself, they captured Remus and handed him captive to Amulius falsly accusing him [of having been in] in Numitor's land, for the purpose of making an attack from there; from there, a gang of youths having been gathered, as an enemy had recently being committing robbery.
'inde eos, collecta iuvenum manu' is a bit tricky..